Feature Request: XMSS and LMS (Stateful HBS) Support

[aidangarske]

Goal
Implement XMSS and HSS/LMS as defined in RFC 8778 to provide wolfCOSE with a competitive advantage. These stateful hash-based signatures are the only PQC schemes currently approved for use in CNSA 2.0 (Commercial National Security Algorithm) for firmware and software signing.

Proposed Implementation

• Define HSS/LMS (-46) and XMSS algorithm identifiers in wolfcose/cose_types.h.
• Integrate with wolfCrypt’s HSS/LMS and XMSS modules (requires WOLFSSL_HSS_LMS / WOLFSSL_XMSS).
• Implement state management callbacks to ensure the private key "index" is safely updated and persisted.
• Expose public API: wc_CoseKey_SetLMS() and wc_CoseKey_SetXMSS().
• Add unit tests using RFC 8778 test vectors.
• Update tools/wolfcose_tool.c to support stateful signing operations.

Optimization Target

• Use pre-hash mode where possible to minimize data passed through the COSE layer.
• Ensure the state-tracking logic is decoupled so users can implement their own non-volatile memory (NVM) storage.

References

Link to RFC: RFC 8778 - HSS/LMS in COSE

Competitor status: Not supported by t_cose or libcose.