ECB CBC WTF

Description

Here you can encrypt in CBC but only decrypt in ECB. That shouldn't be a weakness because they're different modes... right?

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('/ecbcbcwtf/decrypt/<ciphertext>/')
def decrypt(ciphertext):
    ciphertext = bytes.fromhex(ciphertext)

    cipher = AES.new(KEY, AES.MODE_ECB)
    try:
        decrypted = cipher.decrypt(ciphertext)
    except ValueError as e:
        return {"error": str(e)}

    return {"plaintext": decrypted.hex()}


@chal.route('/ecbcbcwtf/encrypt_flag/')
def encrypt_flag():
    iv = os.urandom(16)

    cipher = AES.new(KEY, AES.MODE_CBC, iv)
    encrypted = cipher.encrypt(FLAG.encode())
    ciphertext = iv.hex() + encrypted.hex()

    return {"ciphertext": ciphertext}