# What is SLH-DSA?

SLH-DSA is the hash-based signature standard NIST published in 2024 as FIPS 205. It is based on SPHINCS+, and its security rests only on hash functions.

Also called SPHINCS+, FIPS 205 or hash-based signature.

## How a hash-based signature works

The simplest hash-based signatures can sign only once, because each signature reveals part of the key. SLH-DSA arranges many of them in a tree of Merkle trees, so one key can sign a very large number of messages without keeping track of which parts it used, which is why it is called stateless.

## The trade-off

Its signatures are larger than elliptic-curve ones, from about 8 to 50 kilobytes depending on the parameter set. In return they need no assumption beyond the hash.

## Why it matters now

Justin Drake has suggested that load-bearing signers such as oracles and security councils add a hash-based scheme like SPHINCS to their signatures.

## In Redacted

In bunker mode, nodes sign their approvals and Spending accounts sign their actions with SLH-DSA.

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

## Related terms

- [Post-quantum cryptography](https://redacted.gg/glossary/post-quantum-cryptography.md): Post-quantum cryptography is cryptography built to stay secure against quantum computers, which could break the elliptic curves and RSA that most wallets and websites rely on today.
- [Hash function](https://redacted.gg/glossary/hash-function.md): A hash function turns any input into a short fingerprint of fixed size. The same input always gives the same fingerprint, and nobody can work back from it or find two inputs that share one.
- [ECDSA](https://redacted.gg/glossary/ecdsa.md): ECDSA is the elliptic-curve signature scheme behind most Bitcoin and Ethereum wallets. It proves you hold a private key without showing it, and its security rests on the math of the secp256k1 curve.
- [Merkle tree](https://redacted.gg/glossary/merkle-tree.md): A Merkle tree fingerprints a long list with one short value, the root, so that anyone can prove an item is in the list with a handful of hashes instead of the whole list.

Sources: [FIPS 205, Stateless Hash-Based Digital Signature Standard, NIST](https://csrc.nist.gov/pubs/fips/205/final)
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Page: https://redacted.gg/glossary/slh-dsa/