Feature Request: SLH-DSA (Stateless Hash-Based Signatures)

[aidangarske]

Goal
Implement SLH-DSA (formerly SPHINCS+) as defined in the draft-ietf-cose-sphincs-plus to provide a stateless PQC signing option. This removes the "state management" risk of XMSS/LMS while maintaining the high security of hash-based signatures.

Proposed Implementation

• Define SLH-DSA algorithm identifiers (e.g., SLH-DSA-SHA2-128s: -51) in wolfcose/cose_types.h.
• Map wolfCOSE key types to wolfCrypt’s SLH-DSA implementation.
• Implement the AKP (Algorithm Key Pair) type for flexible PQC key representation.
• Expose public API: wc_CoseKey_SetSLHDSA().
• Add unit tests for both "small" (s) and "fast" (f) parameter sets.
• Update tools/wolfcose_tool.c to support SLH-DSA signing/verification.

Optimization Target

• Guard with #ifdef WOLFCOSE_SLHDSA to manage the significantly larger signature sizes in memory.
• Optimize verification path to utilize wolfCrypt’s hardware-accelerated SHA-256 where available.

References

Link to RFC: Draft - SLH-DSA for JOSE and COSE

Competitor status: No known C-based COSE implementation currently supports this.