Lean4 formalization: hash-based signature security drops to 122 bits under conservative accounting
jedisct1 · x · 2026-11-04
Researchers from Blockstream Research and the Ethereum Foundation published an IACR ePrint preprint formally analyzing the exact bit security of hash-based signatures (XMSS, SLH-DSA) in Lean4.
- For XMSS with 128-bit digests: most adversary work-accounting modes yield 127-bit strong unforgeability; under the most conservative convention (counting only direct random oracle queries), only 122 bits.
- Proofs are in the random oracle model without computational hardness assumptions, with matching attacks; SLH-DSA-128s gets a tight 127-bit bound.
- Motivated by recent autoresearch competitions, the work shows the concrete impact of resource accounting on security notions and aims to guide future challenge designers.
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