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challenge24.ts
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challenge24.ts
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import {mt19937rng, SeededRng} from './challenge21';
import {XORBitArrays} from '../set1/challenge2';
import {BitArray} from '../set1/challenge1';
import * as crypto from 'crypto';
export function generateKeystreamFromRng(bufferLength: number, rand: SeededRng): Buffer {
const keystream = Buffer.alloc(bufferLength);
const bufferBits = bufferLength * 8;
const numOfRandCalls = Math.ceil(bufferBits / 32);
let generated;
let i = 0;
for (; i < numOfRandCalls; i++) {
generated = rand();
if (4*i < bufferLength) keystream[4*i] = (generated & 0xF000) >> 12;
if (4*i + 1 < bufferLength) keystream[4*i + 1] = (generated & 0xF00) >> 8;
if (4*i + 2 < bufferLength) keystream[4*i + 2] = (generated & 0xF0) >> 4;
if (4*i + 3 < bufferLength) keystream[4*i + 3] = generated & 0xF;
}
return keystream;
}
/**
* Encrypt plaintext with MT19937 RNG
* @param plaintext
* @param seed
*/
export function encryptWithMt19937(plaintext: Buffer, seed: number): Buffer {
const rand = mt19937rng(seed, 32);
const keystream = generateKeystreamFromRng(plaintext.length, rand);
return BitArray.toBuffer(XORBitArrays(BitArray.fromBuffer(plaintext), BitArray.fromBuffer(keystream)));
}
/**
* Decrypt ciphertext with MT19937 RNG
* @param ciphertext
* @param seed
*/
export function decryptWithMt19937(ciphertext: Buffer, seed: number): Buffer {
const rand = mt19937rng(seed, 32);
const keystream = generateKeystreamFromRng(ciphertext.length, rand);
return BitArray.toBuffer(XORBitArrays(BitArray.fromBuffer(ciphertext), BitArray.fromBuffer(keystream)));
}
export function encryptKnownPlaintextWithRandomSeed(plaintext: Buffer): { ciphertext: Buffer; seed: number } {
const randomSeed = crypto.randomBytes(2).readUInt16LE();
const randomCount = Math.random() * (10 - 1) + 1;
const random = crypto.randomBytes(randomCount);
return { ciphertext: encryptWithMt19937(Buffer.concat([random, plaintext]), randomSeed), seed: randomSeed } ;
}
export function extractSeed(
knownPlaintext: Buffer,
ciphertext: Buffer,
): number {
let result: Buffer;
for (let i = 0; i < 65536; i++) {
result = decryptWithMt19937(ciphertext, i);
if (result.includes(knownPlaintext)) {
return i;
}
}
throw Error(`Could not extract random seed`);
}
export function generatePasswordToken(): Buffer {
const cur = Date.now();
const token = Buffer.from(`token=value&expires=${cur}`);
return encryptWithMt19937(token, cur);
}
export function isTokenMt19937EncryptedSeededWithTimestamp(
encryptedToken: Buffer
): boolean {
const thirtySecondsAgo = Date.now() - 30 * 1000;
const curDate = Date.now();
let plaintext: Buffer;
for (let i = thirtySecondsAgo; i < curDate; i++) {
plaintext = decryptWithMt19937(encryptedToken, i);
if (plaintext.includes(i.toString(10))) return true;
}
return false;
}