Struct Sphincs 

pub struct Sphincs;

SPHINCS+-SHAKE256-256f-simple (NIST FIPS 205 / SLH-DSA, security level 5, fast variant).

SPHINCS+ is a stateless hash-based signature scheme. Its security rests solely on the collision resistance and preimage resistance of SHAKE256 — no algebraic structure is assumed. This makes it the most conservative signature scheme available.

The f (fast) variant produces larger signatures than the s (small) variant but signs significantly faster. Verification is fast in both variants.

We are using the “simple” (vs. “robust”) variant of SPHINCS+, which is about 3-4x faster than robust. Robust avoids certain assumptions about SHAKE256 at a significant performance cost. In short, simple assumes something called the “random oracle model”: Hash functions are perfect mathematical objects that perfectly map arbitrary-length inputs to fixed-length pseudorandom outputs.

Sizes

Object	Bytes
Public key	64
Secret key	128
Signature	49,856

Pros

• Minimal trust assumption: security reduces entirely to SHAKE256. Even if all lattice-based schemes were broken tomorrow, SPHINCS+ would be unaffected.
• Extremely small public and secret keys (64 B / 128 B) — ideal for constrained devices, key pinning, or any context where key storage is at a premium.
• Stateless: unlike earlier hash-based schemes (XMSS, LMS), no state needs to be maintained between signings, eliminating the risk of catastrophic state reuse.

Cons

• Large signatures (~49 KB with the f variant) — unsuitable for protocols that transmit many signatures frequently (e.g. per-packet authentication). Best suited to infrequent, high-value signings such as root CA certificates, firmware releases, or software packages.
• Signing is slower than ML-DSA, even with the f variant.

Implementations

impl Sphincs

pub fn keypair() -> Result<(PublicKey, SecretKey)>

pub fn sign( message: impl AsRef<[u8]>, secret_key: &SecretKey, ) -> Result<Signature>

pub fn verify( message: impl AsRef<[u8]>, signature: &Signature, public_key: &PublicKey, ) -> Result<()>