SLH-DSA (Stateless Hash-Based Digital Signature Algorithm)
SLH-DSA (Stateless Hash-Based Digital Signature Algorithm), standardised as NIST FIPS 205 in August 2024, is the post-quantum signature standard based on hash functions rather than lattice problems. Where ML-DSA and ML-KEM both rest their security on the hardness of Module Learning With Errors — a relatively young mathematical assumption first formulated in 2005 — SLH-DSA’s security rests exclusively on the collision resistance and preimage resistance of an underlying hash function (SHA-256, SHA-512, or SHAKE, depending on parameter set). Hash function security against quantum computers is well-understood: Grover’s algorithm provides at most a quadratic speedup, which is fully mitigated by doubling output size (SHA-256 remains adequate against classical attacks; SHA-512 provides AES-256-equivalent quantum resistance). The decades-long cryptanalytic confidence in SHA-2 and SHA-3 makes SLH-DSA’s security argument the most conservative available: it requires no new mathematical assumption beyond the hash functions already trusted throughout the entire cryptographic stack.