Create decoder.js

decoder.js

@@ -0,0 +1,58 @@
+const { webcrypto } = require('crypto');
+const sharp = require('sharp');
+
+function pemToDer(pem) {
+    const b64 = pem.replace(/-----BEGIN PRIVATE KEY-----/, '').replace(/-----END PRIVATE KEY-----/, '').replace(/\\n/g, '').replace(/\s/g, '');
+    const binaryDer = atob(b64);
+    const buffer = new ArrayBuffer(binaryDer.length);
+    const bytes = new Uint8Array(buffer);
+    for (let i = 0; i < binaryDer.length; i++) { bytes[i] = binaryDer.charCodeAt(i); }
+    return buffer;
+}
+
+async function decodeFromImageBuffer(imageBuffer, privateKeyPem) {
+    // 1. Extract data from image using Sharp
+    const { data: pixelData } = await sharp(imageBuffer).raw().toBuffer({ resolveWithObject: true });
+    const HEADER_BITS = 32;
+    let headerBinaryString = '';
+    for (let i = 0; headerBinaryString.length < HEADER_BITS; i++) {
+        const pixelIndex = i * 4;
+        if (pixelData.length < pixelIndex + 3) throw new Error("Image too small for header.");
+        headerBinaryString += pixelData[pixelIndex] & 1;
+        if (headerBinaryString.length < HEADER_BITS) headerBinaryString += pixelData[pixelIndex + 1] & 1;
+        if (headerBinaryString.length < HEADER_BITS) headerBinaryString += pixelData[pixelIndex + 2] & 1;
+    }
+    const headerBytes = new Uint8Array(HEADER_BITS / 8);
+    for (let i = 0; i < HEADER_BITS / 8; i++) { headerBytes[i] = parseInt(headerBinaryString.substring(i * 8, (i + 1) * 8), 2); }
+    const dataLengthInBytes = new DataView(headerBytes.buffer).getUint32(0, false);
+    if (dataLengthInBytes === 0) return {};
+
+    const dataLengthInBits = dataLengthInBytes * 8;
+    const bitOffset = HEADER_BITS;
+    let dataBinaryString = '';
+    for (let i = 0; dataBinaryString.length < dataLengthInBits; i++) {
+        const bitPosition = bitOffset + i;
+        const pixelIndex = Math.floor(bitPosition / 3) * 4;
+        const channelOffset = bitPosition % 3;
+        if (pixelData.length < pixelIndex + channelOffset) throw new Error("Image data is corrupt or truncated.");
+        dataBinaryString += pixelData[pixelIndex + channelOffset] & 1;
+    }
+    const cryptoPayload = new Uint8Array(dataLengthInBytes);
+    for (let i = 0; i < dataLengthInBytes; i++) { cryptoPayload[i] = parseInt(dataBinaryString.substring(i * 8, (i + 1) * 8), 2); }
+
+    // 2. Decrypt payload using Web Crypto API
+    const privateKey = await webcrypto.subtle.importKey('pkcs8', pemToDer(privateKeyPem), { name: 'RSA-OAEP', hash: 'SHA-256' }, true, ['decrypt']);
+    const encryptedAesKeyLen = new DataView(cryptoPayload.buffer, 0, 4).getUint32(0, false);
+    let offset = 4;
+    const encryptedAesKey = cryptoPayload.slice(offset, offset + encryptedAesKeyLen);
+    offset += encryptedAesKeyLen;
+    const nonce = cryptoPayload.slice(offset, offset + 12);
+    offset += 12;
+    const ciphertextWithTag = cryptoPayload.slice(offset);
+    const decryptedAesKeyBytes = await webcrypto.subtle.decrypt({ name: 'RSA-OAEP' }, privateKey, encryptedAesKey);
+    const aesKey = await webcrypto.subtle.importKey('raw', decryptedAesKeyBytes, { name: 'AES-GCM', length: 256 }, true, ['decrypt']);
+    const decryptedDataBuffer = await webcrypto.subtle.decrypt({ name: 'AES-GCM', iv: nonce }, aesKey, ciphertextWithTag);
+    return JSON.parse(new TextDecoder().decode(decryptedDataBuffer));
+}
+
+module.exports = { decodeFromImageBuffer };