Crypto is not good as you think

ringzer0ctf

Task: PHP web app issues a base64 Rijndael-256 OFB auth token (user=anonymous|ts=...) with a fixed key/IV; AdminCheck prints the flag when user=admin. Solution: OFB is a stream cipher, so recover the keystream from the known plaintext and XOR in a forged user=admin plaintext with a far-future timestamp to forge an admin token without the key.