Claude Mythos Preview breaks new ground in cryptanalysis: HAWK and reduced-round AES

What happened

Anthropic's Frontier Red Team reported two cryptanalysis results found with the unreleased Claude Mythos Preview. Against HAWK, a third-round NIST post-quantum signature candidate, it found a nontrivial automorphism in the lattice that cuts the expected cost of full key recovery on HAWK-256 from 2^64 to 2^38, halving the effective key size. Against AES-128 reduced to 7 of 10 rounds it found a fingerprinting step it named the Möbius Bridge, giving an attack 200 to 800 times faster than the previous best.

Severity is Medium because neither result touches a deployed system: HAWK is a candidate and is not in production, and the AES attack is on a reduced cipher under a chosen-plaintext model assuming 2^105 plaintexts. The significance is capability, not exposure. HAWK had survived two years of expert review; Mythos improved on it in about 60 hours. Each result cost roughly $100,000 in API spend, and the AES work ran almost autonomously across a billion output tokens on three substantive prompts. Anthropic disclosed the HAWK attack to the scheme's authors in June and to the public NIST mailing list at publication. Unreleased follow-on work includes a practical 13-round LEA key recovery that runs in under an hour on a desktop, a 6-round Serpent-128 recovery, and sub-10x gains against Salsa20, Poseidon and SHA-1.