Forked Tongue

CACTF2026

Task: a TinyGPT decoder-only model plus a byte-level BPE tokenizer whose vocab strings and merges table disagree for high token ids, hiding a second message in model output. Solution: greedy-decode the 5 captured petitions, decode responses both via vocab (the cover story) and via merges entries (the true hidden C2 exfil), then XOR the recovered cipher with a SHAKE-256 keystream derived from the recovered pad.