Discovering Cryptographic Weaknesses

Discovering cryptographic weaknesses with Claude \ Anthropic Frontier Red Team Discovering cryptographic weaknesses with Claude Jul 28, 2026

Summary Using Claude Mythos Preview, researchers at Anthropic have discovered improved ways to attack cryptographic algorithms (the mathematical methods used to keep online data private). The first attack significantly weakens HAWK, a digital signature scheme that was built for a post-quantum world. The second identifies a new way to attack round-reduced AES, the most widely used symmetric cipher. These are substantial research advances, but they do not currently affect any production systems. This post describes both findings in more detail and discusses the implications for cryptography in an age of powerful AI models. Introduction When we launched Claude Mythos Preview, we showed it was able to autonomously find and exploit vulnerabilities in almost every piece of software we pointed it at. This included several major cryptographic libraries—shared collections of code that are used to encrypt data. The vulnerabilities that Claude found in these cryptographic libraries 1 were due to incorrect implementation of the algorithms—that is, errors in how programmers used the algorithms in their code that created opportunities for attackers to break the encryption. Now, we have found that Claude is able to find mathematical flaws in the algorithms themselves . Cryptographic algorithms are a fundamental building block of digital security. For example, when you visit a webpage like https://www.anthropic.com , your browser checks that it is communicating with an authentic website using an algorithm called a digital signature scheme . Later, the traffic between you and the website is encrypted using symmetric ciphers —codes that allow secure data transmission between parties who share an identical key. 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. Flaws in these widely used cryptographic systems could put billions of users’ data at risk. The first result we describe in this post, which was discovered with Claude Mythos Preview, is an improved attack against a digital signature scheme called HAWK . In 2022, the US Government’s National Institute of Standards and Technology (NIST) put out a call for additional cryptographic systems that would remain secure even against quantum computers (which could, if developed, break most of the existing signature schemes in use today). HAWK is one of the third-round candidates under consideration from this call. Despite HAWK having survived two rounds of expert human review over a period of two years, Mythos was able to improve the best-known attack on it in just 60 hours of work—effectively cutting its key strength in half. The second result concerns the Advanced Encryption Standard (AES), a symmetric cipher that was adopted by NIST in 2001 and has received more scrutiny than almost any other encryption algorithm. In order to better understand the robustness of AES, weaker variations of the algorithm are regularly studied in cryptography research; Mythos found a way to break one such weaker version, and eliminated one of the guesses an attacker needs to make, improving the speed of the previous best attacks by 200-800×. To be clear, neither of these results has a practical impact on today’s computer systems; no production software will have to change as a result. HAWK is only a candidate signature scheme and so is not deployed; 2 our second attack is on a reduced version of AES and does not break the full cipher. 3 Nevertheless, both results show the potential for frontier AI models to help discover flaws in important cryptographic algorithms, both before and after real-world deployment. This is cryptography research working as intended: stress-testing algorithms to build trust and ultimately make systems more secure. Mythos Preview achieved these results mostly autonomously and mostly without human intervention. Over the course of a week, one Anthropic researcher worked together with Claude to develop the HAWK attack, and another researcher built a scaffold 4 that allowed Claude to fully autonomously discover the AES attack. 5 Each of the results cost roughly $100,000 in API cost to develop. After seeing these results, we broadened our search and began to discover other attacks. We discuss some of these follow-ups below. In order to make it easier for others to continue studying the cryptanalytic ability of LLMs, we partnered with academics at ETH Zurich, Tel Aviv University, and University of Haifa to build CryptanalysisBench , a benchmark that packages together many cryptographic ciphers and makes it easy for others to evaluate the capabilities of LLMs on this important topic. Throughout the research process, we followed responsible disclosure procedures, and consulted with academics to confirm the validity of our findings. We also shared advance copies with US government and industry partners, and held discussions on the implications of this research. In the case of our HAWK finding, we shared our attack with the authors of HAWK in June and coordinated disclosure to the public NIST mailing list at the same time our results were released. In the rest of this post, we summarize the two findings in further technical detail and briefly describe some of our other recent cryptography results. Full descriptions of the two main findings are provided in two new papers, and we hope to release details for our other findings in the near future. An improved key recovery attack on HAWK Working with Mythos Preview, an Anthropic researcher developed an attack against the HAWK post-quantum digital signature scheme. This attack substantially speeds up the time it would take to break the signature scheme—more technically, it reduces the “effective keysize” by a factor of two. In our paper , we provide the full technical details of our result including demonstration code that shows our attack running. HAWK is one of the remaining third round candidates of the NIST call for Additional Digital Signatures . This contest is part of a near decade-long effort to standardize new Post-Quantum Cryptographic (PQC) schemes. This standardization effort is becoming critical as the horizon to building a cryptographically-relevant quantum computer shrinks and threatens classical cryptography such as RSA or...