Implement RDFC-1.0 canonicalization for VC proof verification

[lokkju]

Problem

The spec requires DataIntegrityProof/eddsa-rdfc-2022 for Verifiable Credential proofs, which uses RDFC-1.0 (RDF Dataset Canonicalization). The current implementation uses JCS (RFC 8785) canonicalization instead.

This means:

• Self-attestation (method: "self") works correctly
• VC attestation (method: "vc") works when VCs are generated by this library, but will not correctly verify VCs signed by third-party issuers using proper RDFC-1.0

What RDFC-1.0 requires

1. JSON-LD expansion of the VC document
2. N-Quads serialization
3. RDFC-1.0 canonicalization (W3C algorithm)
4. Two-step hash: hash of canonical proof options + hash of canonical document
5. Ed25519 signature over the combined hash

Impact

Low for current use cases — the MCP guard only uses self-attestation. This becomes important when:

• Accepting VCs from external identity providers
• Interoperating with other x428 implementations that use proper RDFC-1.0
• Supporting the full eddsa-rdfc-2022 cryptosuite

Implementation notes

• Will need a JSON-LD processor (e.g., jsonld npm package)
• The verifyVcProof function in src/core/verify.ts is the only code that needs to change
• Current JCS-based approach is documented with a comment pointing to this issue
• Consider making the canonicalization strategy pluggable so JCS remains available for testing

References

• W3C RDFC-1.0
• eddsa-rdfc-2022 cryptosuite
• Current code: src/core/verify.ts:verifyVcProof()