CryptoHack Symmetry

题目脚本

from Crypto.Cipher import AES

KEY = ?
FLAG = ?

@chal.route('/symmetry/encrypt/<plaintext>/<iv>/')
def encrypt(plaintext, iv):
    plaintext = bytes.fromhex(plaintext)
    iv = bytes.fromhex(iv)
    if len(iv) != 16:
        return {"error": "IV length must be 16"}

    cipher = AES.new(KEY, AES.MODE_OFB, iv)
    encrypted = cipher.encrypt(plaintext)
    ciphertext = encrypted.hex()

    return {"ciphertext": ciphertext}

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

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

    return {"ciphertext": ciphertext}

这是一个冷门加密算法，现在已经不常见了，加密算法名字为 Output Feedback(OFB)，该算法主要用于产生流密码，具体流程如下：

根据流程，我们发现，明文通过加密后得到密文时，所异或的 KEY 是按照顺序加密生成的密钥流，根据异或的对称性，我们可以通过对密文加密，由此得到我们的明文。

脚本如下

import requests
from tqdm import *
from Crypto.Util.number import *
from pwn import *

requests.adapters.DEFAULT_RETRIES = 100000
s = requests.Session()
s.timeout = (1000.0, 1000.0)

real_c1 = b'admin=False;expi'
fake_c1 = b'admin=True;00000'

encrypt_flag = requests.get(f'http://aes.cryptohack.org/symmetry/encrypt_flag')
result = encrypt_flag.text[15:][:-3]
iv = result[0:32]
ciphertext = result[32:]
print(iv)
print(ciphertext)
encrypt = requests.get(f'http://aes.cryptohack.org/symmetry/encrypt/{ciphertext}/{iv}')
result = bytes.fromhex(encrypt.text[15:][:-3])
print(result)