TraceBash CTF 2026 Crypto Writeup: 4 Challenges Solved (Shared Prime, DH Small-Subgroup, More)
TraceBash CTF 2026 is a Jeopardy-style CTF with a clean, well-curated challenge set. The crypto track has four challenges, three at 100 points and one at 440 points. This writeup covers all four step-by-step. Each challenge is a different shape of cryptographic mistake. state-desync hides a 16-bit-seed stream cipher behind a noisy update function. broken-trust-protocol is a textbook Diffie-Hellman implementation that forgets to validate the peer public value. harmonic-cipher hides an 8-byte XOR key inside an audio file. quantum-echo ships two RSA-1024 public keys that share a 512-bit prime, where a single gcd factors both moduli in milliseconds. None of them requires sage, lattice work, or anything more exotic than a careful read and a small Python brute. All four exist in production code somewhere in the wild. ...