Anthropic's unreleased Claude Mythos Preview model has identified two previously unknown attacks against cryptographic algorithms, including a post-quantum signature scheme currently in the running to become a U.S. federal standard. The findings raise new questions about the security of future blockchain systems like Bitcoin, which are planning upgrades to resist quantum threats.
The first attack targets HAWK, a lattice-based digital signature candidate that was the last of its kind in NIST's third-round post-quantum competition. Claude found a symmetry flaw that reduces the cost of recovering a secret key for the smallest configuration from 2^64 operations to 2^38—a 67 million-fold decrease in required work. Fixing the vulnerability would roughly double HAWK's key size, undermining its main advantage of compact signatures and fast signing. This trade-off is critical for blockchains, where signature size directly affects block space and transaction fees.
The second attack targeted a 7-round research version of AES-128 by inventing a novel technique called a "Möbius Bridge," accelerating the break by 200 to 800 times compared to the previous record set in 2013. Researchers intentionally withheld all five established families of AES cryptanalysis from the model, forcing it to pioneer a new approach. Claude also broke 13 rounds of the LEA cipher, a Korean and ISO lightweight standard, in under an hour. Both AES and LEA results were on weakened versions; production-grade implementations remain secure.
Verification of the AI-generated findings took far longer than discovery—Anthropic researchers spent several hundred hours confirming the AES attack, highlighting a growing bottleneck where human processes struggle to keep pace with model-generated vulnerabilities. The company also released CryptanalysisBench, a benchmark of 191 cipher-breaking tasks where Mythos 5 solved 85.7% of tasks with known solutions but all models scored under 9% against unbroken ciphers.
While there is no immediate threat to Bitcoin's current ECDSA signatures or encrypted internet traffic, the research underscores that AI-driven cryptanalysis could become a rival to quantum computers in forcing cryptographic upgrades. Coin Center's Peter Van Valkenburgh noted the "funniest timeline" scenario where Bitcoin upgrades to a lattice-based post-quantum signature only for AI to break the math first. Anthropic disclosed all findings to algorithm authors and NIST ahead of publication.
