Anthropic researcher, aided by Claude, cracks Classic McEliece challenge instance

matthew_d_green · x · 2026-09-16

Stephen Weis of Anthropic published an IACR paper, written with help from Claude, improving the GIJS key recovery attack against post-quantum candidate Classic McEliece. The large sparse linear-algebra computation in the GIJS distinguisher already contains the secret key; two extraction methods are given, and cost estimates drop to 2^94-2^102 bit operations (vs 2^114-2^124 in GIJS). The team solved the previously unsolved TII challenge instance 253 using two sparse kernel computations (10^7 unknowns, 700 core-hours each). Attacks remain far from practical, but all standardized parameter sets are now no better than NIST Level 1.

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