# What is FIPS 205?

FIPS 205 is the NIST standard for SLH-DSA, a digital signature scheme built on hash functions alone, published in August 2024. It sits beside FIPS 203 and FIPS 204 as one of NIST’s first three post-quantum standards.

Also called Stateless Hash-Based Digital Signature Standard or SLH-DSA standard.

## What the standard specifies

- The SLH-DSA signature algorithm, with its key generation, signing and verification procedures.
- Twelve parameter sets, six built on SHA-2 and six on SHAKE, at NIST security categories 1, 3 and 5.
- Each of the six value sets comes in an s version, for smaller signatures, and an f version, for faster signing.
- Signatures run from 7,856 to 49,856 bytes, and public keys from 32 to 64 bytes.

## Why it uses hashes

NIST selected SPHINCS+ in 2022 as a backup signature scheme that uses different mathematics from the other three selections. FIPS 205 says that its security relies on the presumed difficulty of finding preimages for hash functions, along with several related properties of those functions, and that it is designed to resist attacks from a large-scale quantum computer.

## How it is built

SLH-DSA is built from three hash-based schemes. WOTS+ is a one-time scheme, XMSS is a multi-time scheme built on Merkle hash trees, and FORS is a few-time scheme. A hypertree of XMSS trees authenticates the FORS keys, and the parameter sets are designed for key pairs that sign at most 2^64 messages.

## In Redacted

In bunker mode, nodes sign their approvals and Spending accounts sign their actions with SLH-DSA, the scheme this standard defines.

More: [Bunker mode](https://redacted.gg/docs/using-redacted/bunker-mode.md)

## Related terms

- [SLH-DSA](https://redacted.gg/glossary/slh-dsa.md): SLH-DSA is the hash-based signature standard NIST published as FIPS 205 on 13 August 2024. It is based on SPHINCS+, its security rests on hash functions alone, and the signer keeps no state between signatures.
- [Hash-based signature](https://redacted.gg/glossary/hash-based-signature.md): A hash-based signature proves that the holder of a private key approved a message, and its security rests only on a hash function. Leslie Lamport described the basic idea in 1979, and SLH-DSA, published as FIPS 205 in 2024, is a modern version.
- [ML-DSA](https://redacted.gg/glossary/ml-dsa.md): ML-DSA is the lattice-based digital signature standard that NIST published in August 2024 as FIPS 204. It was derived from CRYSTALS-Dilithium, and its signatures are much shorter than those of the hash-based standard SLH-DSA.
- [ML-KEM](https://redacted.gg/glossary/ml-kem.md): ML-KEM is the key-encapsulation standard that NIST published in August 2024 as FIPS 203. It was derived from CRYSTALS-Kyber, and its security rests on the Module Learning With Errors problem, a lattice problem that is believed to resist quantum attacks.
- [Post-quantum cryptography](https://redacted.gg/glossary/post-quantum-cryptography.md): Post-quantum cryptography is ordinary cryptography designed to stay secure when large quantum computers exist. It replaces the key exchanges and signatures that Shor’s algorithm breaks, such as RSA and elliptic curves, with schemes built on lattices, hash functions or error-correcting codes.

Sources: [FIPS 205, Stateless Hash-Based Digital Signature Standard, NIST, 2024](https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.205.pdf), [NIST releases first 3 finalized post-quantum encryption standards, NIST, August 2024](https://www.nist.gov/news-events/news/2024/08/nist-releases-first-3-finalized-post-quantum-encryption-standards), [NIST announces first four quantum-resistant cryptographic algorithms, NIST, July 2022](https://www.nist.gov/news-events/news/2022/07/nist-announces-first-four-quantum-resistant-cryptographic-algorithms)
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Page: https://redacted.gg/glossary/fips-205/