feat(mcp): add caller nonces and independent attestation verification

[AbdulRashidReshamwala]

User description

Agents could retrieve an attestation document but could not supply their own challenge, and attested: true described successful retrieval rather than independent verification. Require a caller-generated nonce and provide a verifier that runs locally against separately trusted deployment policy.

Changes:

• verify_runtime requires a fresh 32-byte caller nonce, encoded as 64 lowercase hex characters. Missing or invalid challenges fail before any request; there is no server-generated fallback.
• Replace attested with evidence_available and verification.status: "not_performed". Return fixed Google discovery/JWKS references, expected claims, the image-binding algorithm, policy requirements, and claims established by successful verification.
• Add a dependency-free Node verifier for Google RS256 signatures, token times, caller challenge, image hash, pinned image references, audience, project, zone, exact instance name, hardware and secure boot. Optional service-account policy is enforced when configured; current live tokens omit account claims.
• Document the verification steps and what success establishes: Google-signed platform claims and nonce-bound, orchestrator-reported image identities satisfy the approved policy.

Compatibility: this changes the MCP tool contract. Clients must refresh tool discovery, supply a nonce, and consume the replacement output fields. The hosted service is not deployed by this PR.

Validation:

• 78 MCP tests and 20 standalone verifier tests pass; CI now runs the verifier tests.
• Live test: generated and retained the nonce locally, obtained a fresh browser proof, fetched Google keys locally, and verified against policy assembled separately from authenticated Kubernetes pod/node inventory. Signature, nonce, image binding and required policy checks passed; test session cleaned up.
• Independently reviewed by a subagent with no blocking findings.
• Commit SSH-signed under Abdul Rashid's Git identity.

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PR Type

Enhancement, Tests, Documentation

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Description

• Require caller-generated nonce for runtime evidence retrieval

• Replace retrieval-based attestation status with evidence availability

• Add independent Google RS256 proof verifier

• Document trusted policy requirements and verification limits

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Diagram Walkthrough

flowchart LR
  Caller["Caller retains fresh nonce"]
  MCP["MCP retrieves nonce-bound evidence"]
  Proof["Runtime v3 proof"]
  Verifier["Local independent verifier"]
  Policy["Separately trusted policy"]
  Caller -- "supplies nonce" --> MCP
  MCP -- "requests" --> Proof
  Proof -- "returns evidence" --> Verifier
  Policy -- "defines trusted identities" --> Verifier

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File Walkthrough

	Relevant files
Tests	4 files attestation-tool.test.ts Test nonce-required runtime evidence tool contract              +47/-0    popcorn.test.ts Test caller nonce evidence retrieval behavior                        +30/-8    oss-ci.yml Run independent attestation verifier tests                              +3/-0      verify.test.mjs Test signature claims policy and bindings                                +62/-0
Enhancement	4 files attestation.ts Define nonce validation and verification guidance                +49/-0    popcorn.ts Retrieve runtime proofs using caller nonce                              +4/-4      tools.ts Update runtime verification tool schema and output              +21/-7    verify.mjs Add local Google attestation proof verifier                            +122/-0
Documentation	2 files README.md Document independent proof verification workflow                  +100/-0  README.md Document breaking runtime verification contract                    +14/-8

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Approved by Reclaim PR Agent after an automated GPT-5.6 Terra review.

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Approved by Reclaim PR Agent after an automated GPT-5.6 Terra review.

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Approved by Reclaim PR Agent after an automated GPT-5.6 Terra review.