interop: replay independent Measurement Capsule verifier on current main - #197

Purpose

Clean replay of #195 onto current main, now extended only where external evidence justified it.

Independent implementation

Implement the identifier and batch-integrity portions of draft-templeman-scitt-measurement-capsule-00 without importing the CSOAI prototype builder/verifier:

• RFC 8785/JCS canonicalization via the independent Trail of Bits rfc8785 package;
• capsule-id recomputation with capsule_id excluded;
• exact canonical JSONL line checks;
• no-decision/no-authority boundary checks;
• digest-only source admission;
• UNCHECKABLE preservation;
• RFC 6962/9162 Merkle Tree Hash over sorted capsule-id bytes;
• duplicate refusal;
• expected root/count checks.

Real external replay already reached

Against public CSOAI repository commit
85df6bc2ed76450c171d6982bfd39e94d80982d2,
CounterProof now pins the published claim_watch/leaves.json value:

007b0e5ccbb3f86ac587d645dd8df74c689765267eadc09736eb58bf8be95314

and independently recomputes:

SHA-256(0x00 || capsule_id_bytes)

to the published batch root:

5e6440b6f9fbccdc2dca66aff3c79c02f649ccb56cd795d0f043b651dd50c32e.

That is a real external Merkle replay, not a synthetic vector.

Full external replay gate

This PR also adds a dedicated CI job that fetches the exact commit-pinned public:

public/measurement-capsules/v0.2/claim_watch/capsules.jsonl.gz

from CSOAI-ORG/councilof-ai, verifies the published gzip SHA-256, decompresses it, recomputes the capsule id from the real canonical capsule bytes, and requires the resulting batch root/count to match the public record.

Claim boundary

The leaf/root replay alone does not establish the draft's full external-batch experiment because the capsule id is still upstream-provided.

The stronger claim is allowed only if the dedicated external replay CI job passes against the real gzip bytes.

Even a full byte/root match proves binding and reproducibility only. It does not prove that the underlying measurement is true, independent, unbiased, authorised, or endorsed by SCITT/IETF.