CTRIME

Description

There may be a lot of redundancy in our plaintext, so why not compress it first?

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
from Crypto.Util import Counter
import zlib


KEY = ?
FLAG = ?


@chal.route('/ctrime/encrypt/<plaintext>/')
def encrypt(plaintext):
    plaintext = bytes.fromhex(plaintext)

    iv = int.from_bytes(os.urandom(16), 'big')
    cipher = AES.new(KEY, AES.MODE_CTR, counter=Counter.new(128, initial_value=iv))
    encrypted = cipher.encrypt(zlib.compress(plaintext + FLAG.encode()))

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