Python/verifier.py

# -*- coding: utf-8 -*-

# Copyright 2018 New Vector Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from __future__ import absolute_import

import logging
import time

from twisted.internet import defer
from unpaddedbase64 import decode_base64
import signedjson.sign
import signedjson.key
from signedjson.sign import SignatureVerifyException

from sydent.http.httpclient import FederationHttpClient


logger = logging.getLogger(__name__)


class NoAuthenticationError(Exception):
    """
    Raised when no signature is provided that could be authenticated
    """
    pass


class Verifier(object):
    """
    Verifies signed json blobs from Matrix Homeservers by finding the
    homeserver's address, contacting it, requesting its keys and
    verifying that the signature on the json blob matches.
    """
    def __init__(self, sydent):
        self.sydent = sydent
        # Cache of server keys. These are cached until the 'valid_until_ts' time
        # in the result.
        self.cache = {
            # server_name: <result from keys query>,
        }

    @defer.inlineCallbacks
    def _getKeysForServer(self, server_name):
        """Get the signing key data from a homeserver.

        :param server_name: The name of the server to request the keys from.
        :type server_name: unicode

        :return: The verification keys returned by the server.
        :rtype: twisted.internet.defer.Deferred[dict[unicode, dict[unicode, unicode]]]
        """

        if server_name in self.cache:
            cached = self.cache[server_name]
            now = int(time.time() * 1000)
            if cached['valid_until_ts'] > now:
                defer.returnValue(self.cache[server_name]['verify_keys'])

        client = FederationHttpClient(self.sydent)
        # BUG: CWE-770 Allocation of Resources Without Limits or Throttling
        # result = yield client.get_json("matrix://%s/_matrix/key/v2/server/" % server_name)
        # FIXED: 
        result = yield client.get_json("matrix://%s/_matrix/key/v2/server/" % server_name, 1024 * 50)
        if 'verify_keys' not in result:
            raise SignatureVerifyException("No key found in response")

        if 'valid_until_ts' in result:
            # Don't cache anything without a valid_until_ts or we wouldn't
            # know when to expire it.
            logger.info("Got keys for %s: caching until %s", server_name, result['valid_until_ts'])
            self.cache[server_name] = result

        defer.returnValue(result['verify_keys'])

    @defer.inlineCallbacks
    def verifyServerSignedJson(self, signed_json, acceptable_server_names=None):
        """Given a signed json object, try to verify any one
        of the signatures on it

        XXX: This contains a fairly noddy version of the home server
        SRV lookup and signature verification. It does no caching (just
        fetches the signature each time and does not contact any other
        servers to do perspective checks).

        :param acceptable_server_names: If provided and not None,
        only signatures from servers in this list will be accepted.
        :type acceptable_server_names: list[unicode] or None

        :return a tuple of the server name and key name that was
        successfully verified.
        :rtype: twisted.internet.defer.Deferred[tuple[unicode]]

        :raise SignatureVerifyException: The json cannot be verified.
        """
        if 'signatures' not in signed_json:
            raise SignatureVerifyException("Signature missing")
        for server_name, sigs in signed_json['signatures'].items():
            if acceptable_server_names is not None:
                if server_name not in acceptable_server_names:
                    continue

            server_keys = yield self._getKeysForServer(server_name)
            for key_name, sig in sigs.items():
                if key_name in server_keys:
                    if 'key' not in server_keys[key_name]:
                        logger.warn("Ignoring key %s with no 'key'")
                        continue
                    key_bytes = decode_base64(server_keys[key_name]['key'])
                    verify_key = signedjson.key.decode_verify_key_bytes(key_name, key_bytes)
                    logger.info("verifying sig from key %r", key_name)
                    signedjson.sign.verify_signed_json(signed_json, server_name, verify_key)
                    logger.info("Verified signature with key %s from %s", key_name, server_name)
                    defer.returnValue((server_name, key_name))
            logger.warn(
                "No matching key found for signature block %r in server keys %r",
                signed_json['signatures'], server_keys,
            )
        logger.warn(
            "Unable to verify any signatures from block %r. Acceptable server names: %r",
            signed_json['signatures'], acceptable_server_names,
        )
        raise SignatureVerifyException("No matching signature found")

    @defer.inlineCallbacks
    def authenticate_request(self, request, content):
        """Authenticates a Matrix federation request based on the X-Matrix header
        XXX: Copied largely from synapse

        :param request: The request object to authenticate
        :type request: twisted.web.server.Request
        :param content: The content of the request, if any
        :type content: bytes or None

        :return: The origin of the server whose signature was validated
        :rtype: twisted.internet.defer.Deferred[unicode]
        """
        json_request = {
            "method": request.method,
            "uri": request.uri,
            "destination_is": self.sydent.server_name,
            "signatures": {},
        }

        if content is not None:
            json_request["content"] = content

        origin = None

        def parse_auth_header(header_str):
            """
            Extracts a server name, signing key and payload signature from an
            authentication header.

            :param header_str: The content of the header
            :type header_str: unicode

            :return: The server name, the signing key, and the payload signature.
            :rtype: tuple[unicode]
            """
            try:
                params = header_str.split(u" ")[1].split(u",")
                param_dict = dict(kv.split(u"=") for kv in params)

                def strip_quotes(value):
                    if value.startswith(u"\""):
                        return value[1:-1]
                    else:
                        return value

                origin = strip_quotes(param_dict["origin"])
                key = strip_quotes(param_dict["key"])
                sig = strip_quotes(param_dict["sig"])
                return origin, key, sig
            except Exception:
                raise SignatureVerifyException("Malformed Authorization header")

        auth_headers = request.requestHeaders.getRawHeaders(u"Authorization")

        if not auth_headers:
            raise NoAuthenticationError("Missing Authorization headers")

        for auth in auth_headers:
            if auth.startswith(u"X-Matrix"):
                (origin, key, sig) = parse_auth_header(auth)
                json_request["origin"] = origin
                json_request["signatures"].setdefault(origin, {})[key] = sig

        if not json_request["signatures"]:
            raise NoAuthenticationError("Missing X-Matrix Authorization header")

        yield self.verifyServerSignedJson(json_request, [origin])

        logger.info("Verified request from HS %s", origin)

        defer.returnValue(origin)