# Claude AI finds new cryptographic weaknesses, Anthropic says

> Anthropic's Claude Mythos found new attacks on several cryptographic schemes.

*The model cut HAWK's effective key size in half and broke reduced-round AES hundreds of times faster than known methods.*

By Behzad Hosseini · FeaturedDaily
Canonical: https://featureddaily.com/news/claude-ai-finds-new-cryptographic-weaknesses-anthropic-says

**What happened:** Anthropic said its Claude Mythos Preview model discovered new attacks against several cryptographic schemes, according to [research published 28 July 2026](https://www.anthropic.com/research/discovering-cryptographic-weaknesses).

**The numbers:** Against the post-quantum signature scheme HAWK, the model halved the effective key size after about 60 hours of work and roughly $100,000 in API cost. Against AES reduced to 7 rounds, it produced an attack 200 to 800 times faster than the best previously known, using 3 days of autonomous work, 1 billion output tokens and about $100,000.

**The details:** The model also broke LEA reduced to 13 rounds with a practical attack needing fewer than 2^30 plaintexts in under an hour, and achieved full key recovery on Serpent-128 reduced to 6 rounds. It made only small improvements, under 10 times, against Salsa20, Poseidon and SHA-1.

**The catch:** Full-strength AES was not broken. Reduced-round variants are how cryptographers measure a cipher's safety margin, not real-world vulnerabilities on their own.

**Who's affected:** Human researchers still had to check the work. Verifying the AES result took two researchers nearly a month, several hundred hours, during which they had to learn cryptography themselves. The HAWK result was checked by one researcher with a theoretical computer science background.

**The context:** The HAWK authors were notified in June. Disclosure of the findings was coordinated with NIST, the US government and industry partners.

**In their words:** Anthropic said: "Without secure cryptographic systems like these, your email, online banking, and other internet use would be open to cybercriminals, who could intercept or modify your communications."

**Why it matters:** The results show an AI model producing original findings in one of the most demanding fields of mathematics and security, rather than just applying known techniques.

**What's next:** Anthropic gave no timeline for further cryptographic testing beyond the disclosed coordination with NIST and industry partners.

## Key takeaways

- HAWK's effective key size cut by 50% for about $100,000
- Reduced-round AES broken 200–800 times faster than before
- Full AES remains unbroken; disclosure coordinated with NIST

## Sources

- [Discovering cryptographic weaknesses with Claude](https://www.anthropic.com/research/discovering-cryptographic-weaknesses) — Anthropic, 2026-07-28