π Last month, our team took on a challenge called crypto-numerology.
At first glance, it looked solid: a stream cipher modeled after ChaCha20. It had proper constants, key/nonce structure, and ciphertext that looked convincingly random.
But there was one critical detail.
π It only used one round of mixing.
That one shortcut changed everything.
With a known key and a few plaintext/ciphertext pairs, we could fully recover keystream blocks. From there, it only took a small brute-force over a 32-bit counter to reveal the flag.
π What struck me most was how realistic this failure felt.
“One round should be fine.”“Nobody will reuse this nonce.”“It’s just for internal use.”
Security doesn't usually break in dramatic ways - it rots quietly, through shortcuts and assumptions that go unchallenged until it’s too late.