**crypto-numerology (Crypto, 50 pts)**

This challenge presents a custom stream cipher inspired by ChaCha20 - but with a twist: it uses only a single round of mixing operations.

Given a known key and a batch of plaintext/ciphertext pairs, the task is to decrypt a secret message.

By analyzing the known samples, we recovered the keystream and brute-forced the correct counter value using a fixed nonce. A great exercise in exploiting weakened cipher designs.

**Flag:** `CTF{w3_aRe_g0Nn@_ge7_MY_FuncKee_monkey_!!}`

See the [full writeup](https://github.com/CTF-STeam/ctf-writeups/tree/master/2025/GoogleCTF/crypto-numerology) for detailed analysis and solution.

Original writeup (https://github.com/CTF-STeam/ctf-writeups/tree/master/2025/GoogleCTF/crypto-numerology).