Feature Request: Counter Signature Support (RFC 9052)

[aidangarske]

Goal
Implement Counter Signatures as defined in RFC 9052. This enables "signatures on signatures," a requirement for supply chain security (SCITT).

Proposed Implementation

• Define Countersignature0 and Countersignature types in wolfcose/cose_types.h.
• Add logic to wolfCose_Sign_Internal to handle secondary signature headers.
• Expose public API: wc_Cose_AddCounterSignature().
• Add unit tests in tests/ for nested signature verification.
• Update tools/wolfcose_tool.c to allow adding countersignatures to existing blobs.

Optimization Target

• Ensure header parsing logic does not exceed 2 KB of stack space.
• Support "In-Place" countersigning to minimize buffer copies.

References

Link to RFC: RFC 9052 - COSE Structures and Algorithms

Competitor status: Explicitly listed as "Not Supported" by t_cose