Anthropic's Claude Mythos Preview model uncovered a flaw in the proposed HAWK digital-signature scheme that effectively halves its smallest key strength, undercutting one of the candidates considered for replacing current web and banking signatures in a post-quantum world.

The AI-driven attack took about 60 hours and roughly $100,000 in computing costs. It reduced the work needed to break HAWK's smallest parameter set from about 2^64 operations to 2^38, while making larger key sizes used as compensation far less appealing.

Although today's bitcoin and ether signatures are unaffected, the findings add to warnings that classical cryptanalytic attacks are improving quickly, even as bitcoin and other networks debate when and how to migrate to quantum-resistant cryptography.

Anthropic said earlier this week that Claude Mythos Preview had found an attack that halves the effective key strength of HAWK, a proposed replacement for the digital signatures used in online banking and web payments. The work came after about 60 hours of compute time and approximately $100,000 in costs, and it targeted an algorithm that had survived two years and two rounds of expert human review. NIST, the U.S. government agency whose cryptography standards are adopted across federal systems and commercial software worldwide, has been evaluating HAWK as part of an ongoing multi-year selection process for additional post-quantum signature algorithms.

Digital signature schemes such as HAWK are designed to prove that a message or data set came from whoever holds the relevant private key. Bitcoin uses digital signatures every time a coin moves, and websites rely on them whenever a browser displays a padlock. The versions in use today are expected to fail against quantum computers, and HAWK is one of the candidates under review to replace them. It has not been publicly deployed anywhere.

The research does not affect bitcoin or ether as they are used today. Both networks secure transactions with elliptic curve signatures, and neither of Anthropic's attacks targeted those schemes. HAWK is also not one of the algorithms bitcoin would migrate to.

BIP-360, the proposal for bitcoin quantum-resistant addresses, specifies three algorithms that NIST has already standardized and includes multiple options so users have fallbacks if one is later broken by quantum or classical advances.

What has changed is the pace of classical cryptanalysis. BIP-361, the companion proposal that would freeze more than a third of bitcoin's supply, says the migration window is closing because cryptographic attacks are improving by as much as 20-fold. Anthropic's results fit that trend, with AI behind the breakthrough — a tool that produced in days what two years of expert human review had missed.

For HAWK's smallest parameter set, Anthropic said the expected cost of recovering a key fell from about 2^64 operations to 2^38. Larger keys remain impractical to attack, but increasing key sizes to compensate removes much of what made HAWK attractive in the first place.

Anthropic disclosed the attack to HAWK's authors in June and coordinated publication with NIST's public mailing list.

The company also reported a second result: improved attacks on a deliberately weakened version of AES, the cipher widely used across the industry to encrypt wallet files, by a factor of 200 to 800.

For crypto developers, Anthropic said the model produced smaller gains, of less than tenfold, against Poseidon, the hash function that underpins many zero-knowledge proof systems, including those used to secure rollups and privacy protocols.

Claude, Anthropic's consumer AI tool, initially refused the AES challenge, telling researchers it was "genuinely hard" and that there was "nothing easy to find." After three short prompts, however, the model generated a billion output tokens over three days and found the improvement. Two Anthropic researchers then spent nearly a month verifying it.

The timing is notable. Privacy network Zcash activated an upgrade on Tuesday that introduced a new shielded pool designed to remain recoverable if quantum computers become available.

The systems intended to support that transition are now being stress-tested by something that operates faster than the people designing them.