SPHINCS+

Definition

A NIST-standardized (FIPS 205) stateless hash-based post-quantum digital signature scheme that derives its security solely from hash function properties, providing a security hedge against potential weaknesses in lattice-based approaches.

Technical Details

SPHINCS+ (standardized as SLH-DSA in FIPS 205) constructs signatures from Merkle tree structures using only hash functions (SHA-256 or SHAKE variants), whose post-quantum security is well-established. Unlike lattice-based schemes, its security does not depend on unproven hardness assumptions beyond hash function collision resistance. Trade-offs: larger signature sizes (8KB-50KB) and slower performance than Dilithium, but it provides algorithmic diversity.

Practical Usage

Organizations that need maximum security assurance against unknown attacks on lattice assumptions should use SPHINCS+ as a complementary or alternative signature scheme. It is particularly appropriate for long-lived signatures (code signing, document signing) where signature size is less critical than long-term security.

Examples

• NIST published FIPS 205 (SLH-DSA based on SPHINCS+) in August 2024 as an alternative post-quantum signature standard.
• A critical infrastructure operator uses SPHINCS+ for firmware signing to hedge against potential future lattice cryptanalysis.
• A hybrid signature scheme combines Dilithium + SPHINCS+ signatures on software releases for defense-in-depth against algorithm-specific attacks.

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