Lazy CBC

Description

I'm just a lazy dev and want my CBC encryption to work. What's all this talk about initialisations vectors? Doesn't sound important.

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 AES


KEY = ?
FLAG = ?


@chal.route('/lazy_cbc/encrypt/<plaintext>/')
def encrypt(plaintext):
    plaintext = bytes.fromhex(plaintext)
    if len(plaintext) % 16 != 0:
        return {"error": "Data length must be multiple of 16"}

    cipher = AES.new(KEY, AES.MODE_CBC, KEY)
    encrypted = cipher.encrypt(plaintext)

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


@chal.route('/lazy_cbc/get_flag/<key>/')
def get_flag(key):
    key = bytes.fromhex(key)

    if key == KEY:
        return {"plaintext": FLAG.encode().hex()}
    else:
        return {"error": "invalid key"}


@chal.route('/lazy_cbc/receive/<ciphertext>/')
def receive(ciphertext):
    ciphertext = bytes.fromhex(ciphertext)
    if len(ciphertext) % 16 != 0:
        return {"error": "Data length must be multiple of 16"}

    cipher = AES.new(KEY, AES.MODE_CBC, KEY)
    decrypted = cipher.decrypt(ciphertext)

    try:
        decrypted.decode() # ensure plaintext is valid ascii
    except UnicodeDecodeError:
        return {"error": "Invalid plaintext: " + decrypted.hex()}

    return {"success": "Your message has been received"}