FIPS 205 — SLH-DSA (Hash-Based Signatures)
What It Is in One Sentence
SLH-DSA creates signatures using only hash functions — no new mathematical assumptions. If SHA-2 and SHA-3 remain secure, SLH-DSA remains secure.
The Problem
All other PQC signatures (ML-DSA, FN-DSA) rely on lattice problems:
- Module-LWE for ML-KEM and ML-DSA
- NTRU-SIS for FN-DSA
These are well-studied but relatively new (20 years of cryptanalysis vs. 40+ for RSA/factoring). What if a breakthrough in lattice algorithms renders them weak?
SLH-DSA provides a conservative fallback — its security depends only on hash functions, which have been the backbone of cryptography since the 1970s.
How It Works (High Level)
SLH-DSA combines three building blocks:
Rendering diagram...
| Building Block | Role | Security Assumption |
|---|---|---|
| WOTS+ | Sign one message with one key | Hash function preimage resistance |
| FORS | Sign a few messages with one key | Hash function collision resistance |
| Hypertree | Organise millions of keys into a tree | Hash function collision resistance |
Quick Reference
| Parameter set | Hash | Security | Public key | Signature |
|---|---|---|---|---|
| SLH-DSA-SHA2-128s | SHA-256 | Level 1 | 32 B | 7,856 B |
| SLH-DSA-SHA2-128f | SHA-256 | Level 1 | 32 B | 17,064 B |
| SLH-DSA-SHA2-192s | SHA-256 | Level 3 | 48 B | 16,224 B |
| SLH-DSA-SHAKE-256s | SHAKE256 | Level 5 | 64 B | 29,792 B |
s = small (smaller signatures, slower signing). f = fast (larger signatures, faster signing).
Articles in This Section
- 01 — Why Hash-Based — Conservative security and trust minimisation
- 02 — Merkle Trees — The data structure behind hash-based crypto
- 03 — One-Time Signatures (WOTS+) — Signing once, securely
- 04 — Few-Time Signatures (FORS) — Signing a few times per key
- 05 — The Hypertree — Scaling to millions of signatures
- 06 — Stateless Property — Why no key state tracking needed
- 07 — Size vs. Speed — The fundamental trade-off
- 08 — When to Use — Trust anchor and fallback scenarios
Resources
- NIST FIPS 205 PDF
- SPHINCS+ specification
- Bernstein et al., "SPHINCS+ — Submission to the NIST post-quantum project" (2020)