Triple DES

Description

Data Encryption Standard was the forerunner to AES, and is still widely used in some slow-moving areas like the Payment Card Industry. This challenge demonstrates a strange weakness of DES which a secure block cipher should not have.

Help

This page offers a convenient way for you to interact with the challenge functions. You can also use GET requests to send and receive data directly from the listed routes/endpoints if you wish. For more information see the FAQ.

Your aim is to recover the FLAG value. Once you have it, submit it on the CryptoHack Symmetric Ciphers page.

Source

from Crypto.Cipher import DES3
from Crypto.Util.Padding import pad


IV = os.urandom(8)
FLAG = ?


def xor(a, b):
    # xor 2 bytestrings, repeating the 2nd one if necessary
    return bytes(x ^ y for x,y in zip(a, b * (1 + len(a) // len(b))))



@chal.route('/triple_des/encrypt/<key>/<plaintext>/')
def encrypt(key, plaintext):
    try:
        key = bytes.fromhex(key)
        plaintext = bytes.fromhex(plaintext)
        plaintext = xor(plaintext, IV)

        cipher = DES3.new(key, DES3.MODE_ECB)
        ciphertext = cipher.encrypt(plaintext)
        ciphertext = xor(ciphertext, IV)

        return {"ciphertext": ciphertext.hex()}

    except ValueError as e:
        return {"error": str(e)}


@chal.route('/triple_des/encrypt_flag/<key>/')
def encrypt_flag(key):
    return encrypt(key, pad(FLAG.encode(), 8).hex())