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Context / Closes #135. Defines a layered signature architecture: SHARED FOUNDATION ├── Canonicalization: JCS (RFC 8785) ├── Algorithms: ES256 (required), ES384, ES512 ├── Key Format: JWK (RFC 7517) ├── Key Discovery: signing_keys[] in /.well-known/ucp └── Replay Protection: idempotency-key (business layer) │ ├── REST BINDING (RFC 9421) │ Headers: Signature, Signature-Input, UCP-Content-Digest-JCS │ └── MCP BINDING (RFC 7515 Appendix F) Fields: meta.signature, meta.idempotency-key, meta.ucp-agent JCS canonicalization (RFC 8785): - Deterministic JSON serialization before signing - Avoids whitespace/key-ordering verification failures - Custom header `UCP-Content-Digest-JCS` (not RFC 9530 Content-Digest) because we hash canonicalized JSON, not raw bytes Transport-specific formats: - REST uses RFC 9421 HTTP Message Signatures (modern standard) - MCP uses detached JWS (RFC 7515 Appendix F) in meta.signature - Both sign the full message; MCP excludes only meta.signature field Changes to OpenAPI: - `Request-Signature` header → `Signature` + `Signature-Input` headers - `X-Detached-JWT` response header → `Signature` + `Signature-Input` - `UCP-Content-Digest-JCS` header for body digest Other updates: - checkout-rest.md, checkout-mcp.md: Added Message Signing sections - order.md: Rewrote webhook signing to use RFC 9421 - ap2-mandates.md: Now references signatures.md for crypto primitives - openrpc.json: Added signature field to meta, added meta to cart methods - mkdocs.yml: Added signatures.md to nav and llmstxt sections
igrigorik
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drewolson-google
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Feb 5, 2026
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drewolson-google
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Generally looks very good to me. I've included a few small comments.
- Separate "Implementation requirements" (MUST verify ES256) from
"Usage guidance" (SHOULD use ES256 for compatibility)
- Removed "Signers: use newest key" - no way to determine key age
from JWK spec, and verifiers accept any key in signing_keys[]
| For signature computation over JSON payloads, implementations **MUST** use | ||
| **JSON Canonicalization Scheme (JCS)** as defined in | ||
| [RFC 8785](https://datatracker.ietf.org/doc/html/rfc8785). | ||
| All JSON payloads **MUST** be canonicalized using **JSON Canonicalization |
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AIUI canonicalized JSON is generally considered much more problematic than signing the body bytes of the serialized JSON. I would highly recommend we avoid doing so.
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Context / Closes #135. Defines a layered signature architecture:
JCS canonicalization (RFC 8785):
UCP-Content-Digest-JCS(not RFC 9530 Content-Digest)because we hash canonicalized JSON, not raw bytes
Transport-specific formats:
Changes to OpenAPI:
Request-Signatureheader →Signature+Signature-InputheadersX-Detached-JWTresponse header →Signature+Signature-InputUCP-Content-Digest-JCSheader for body digestOther updates:
Checklist