SPHINCS+ (SLH-DSA)

SPHINCS+ is a stateless hash-based signature scheme standardized as SLH-DSA.

Overview

• Type: Digital Signature Algorithm
• Standard: FIPS 205 (SLH-DSA)
• Security basis: Hash functions only

Key properties

• Stateless: No state to manage between signatures
• Conservative: Security relies only on hash functions
• Large signatures: Trade-off for simplicity

Parameter sets

Variant	Security	Public key	Signature
SPHINCS+-128s	Level 1	32 bytes	7856 bytes
SPHINCS+-128f	Level 1	32 bytes	17088 bytes
SPHINCS+-256s	Level 5	64 bytes	29792 bytes
SPHINCS+-256f	Level 5	64 bytes	49856 bytes

s = small (slower, smaller signatures) f = fast (faster, larger signatures)

QNSP usage

• Use cases: Long-term signatures, archival
• Fallback: When lattice assumptions are questioned

Advantages

• Minimal cryptographic assumptions
• Well-understood security
• No complex mathematics

Disadvantages

• Very large signatures
• Slower than lattice-based schemes

When to use SPHINCS+

• Maximum conservatism required
• Long-term document signatures
• Regulatory requirements for hash-based
• Backup/fallback algorithm