Anthropic’s new benchmark finds frontier LLMs can break 65%–86% of tier-1 ciphers

AnthropicAI · x · 2026-07-29

Anthropic says it worked with researchers at ETH Zurich, Tel Aviv University, and the University of Haifa on CryptanalysisBench, a benchmark for testing whether LLMs can do cryptanalysis.

The benchmark includes 191 tasks across six families of cryptographic primitives, drawn mainly from four NIST standardization competitions.
It spans three tiers: primitives with known practical breaks, stronger primitives tested at full strength and on scaled-down variants, and a frontier challenge set.
Across five frontier models — Claude Opus 4.8, Sonnet 5, Mythos 5, GPT 5.5, and open-weights GLM 5.2 — the paper reports 65%–86% breaks on Tier 1 schemes, plus 6–12 Tier-2 schemes at full strength and 24–61 on scaled-down variants.
The authors also say models discovered novel cryptanalysis, including a key-recovery attack against a flaw in SpoC AEAD and an error in KINDI’s published CCA-security proof.

Anthropic links the benchmark to two new papers describing the attacks in detail.

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Anthropic’s new benchmark finds frontier LLMs can break 65%–86% of tier-1 ciphers

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