Anthropic's Claude Mythos Cracks Two Encryption Systems in 60 Hours
Claude Mythos Preview cracked a post-quantum signature scheme and accelerated an AES attack by up to 800x, doing in days what human experts missed for years
- Claude Mythos Preview found a previously unknown attack on HAWK, a post-quantum signature candidate, cutting its effective key strength in half in just 60 hours.
- The model also improved attacks on 7-round AES by 200-800x using a novel technique it invented called the Möbius Bridge.
- Both results were achieved mostly autonomously, with minimal human prompting, at roughly $100,000 in API cost each.
- Anthropic released CryptanalysisBench, a 191-task benchmark across 6 cipher families to track LLM cryptanalysis capabilities over time.
- Neither attack breaks production systems: HAWK is undeployed, and the AES result targets a reduced 7-round variant, not the full cipher.
- Demonstration code for the HAWK attack is on GitHub; Anthropic also published Claude's raw chain-of-thought during the AES discovery.
Cryptography has always been a game of adversarial review: propose an algorithm, let the world attack it, and trust what survives. New research from Anthropic suggests AI is about to dramatically accelerate that game. Using Claude Mythos Preview, Anthropic researchers found meaningful weaknesses in two cryptographic systems: one a post-quantum signature candidate still under government review, the other a reduced variant of the most battle-tested cipher in existence.
Why cryptography, why now
Cryptographic algorithms are the foundation of digital security. When you visit a webpage, your browser checks authenticity using a digital signature scheme and encrypts traffic using a symmetric cipher. Flaws in widely deployed systems could put billions of users' data at risk. Until recently, finding those flaws required years of expert human effort. Anthropic set out to test whether a frontier AI model could compress that timeline.
When Anthropic launched Claude Mythos Preview, it could already autonomously find and exploit vulnerabilities in major cryptographic libraries. Those vulnerabilities stemmed from incorrect implementation: errors in how programmers used the algorithms in their code. This new work goes a level deeper, targeting mathematical flaws in the algorithms themselves.
Two targets, two results
The first target was HAWK, a digital signature scheme designed to resist quantum computers. In 2022, NIST solicited additional cryptographic systems that would remain secure even as quantum hardware matures, since quantum computers could break most signature schemes in use today. HAWK is a third-round candidate. It had survived two years of expert human review. Mythos improved the best-known attack on it in 60 hours, effectively cutting its key strength in half.
The second target was a reduced-round variant of AES (Advanced Encryption Standard), the symmetric cipher securing most encrypted internet traffic. AES encrypts data by repeatedly applying the same round function: AES-128 runs 10 rounds. The attack targets a modified version with 7 of those 10 rounds. Academics study round-reduced ciphers to develop attack techniques that might generalize to the full cipher. Mythos extended a long line of research using a technique called a meet-in-the-middle attack, which trades storage for speed: by caching intermediate calculations in a lookup table, you can cut runtime significantly.