fix(provenance,conformance): reject private JWK members and wrap _pubkey_from_jwk errors (#535)

[drk-ram]

What this changes

Closes #535.

Fixes three gaps around RFC 7517 / RFC 7518 private key material ("d", "p", "q", "dp", "dq", "qi", "k", GHSA-vc4p-h84j-7qxj) and JWK error handling across provenance.verify_record and conformance/src/trace_tests/modules/tr_sig.py:

1. src/agentrust_trace/provenance.py (verify_record):
   • Rejects any record["cnf"]["jwk"] or trusted_jwk carrying private/secret JWK members (_JWK_PRIVATE_PARAMS = frozenset({"d", "p", "q", "dp", "dq", "qi", "k"})) with ProvenanceError, naming only the offending member key names and never echoing secret values.
   • Checks trusted_jwk for private key material before consulting revocation so a call with revocation supplied does not invoke the revocation store or surface a thumbprint/revocation error before rejecting a trusted_jwk carrying private key material.
   • Wraps _pubkey_from_jwk(trusted_jwk) in try ... except ValueError as exc: raise ProvenanceError(f"the record's embedded key is unusable: {exc}") from exc so non-Ed25519 or malformed JWKs (RSA, EC, X25519, short x, or padded/non-canonical base64url x) raise ProvenanceError instead of leaking a bare ValueError.
2. conformance/src/trace_tests/modules/tr_sig.py (check and check_cmcp_runtime):
   • Defines _JWK_PRIVATE_PARAMS = frozenset({"d", "p", "q", "dp", "dq", "qi", "k"}) (matching tr_env.py and agentrust_trace.models).
   • In check_cmcp_runtime, rejects any trace.cnf.jwk carrying a member of _JWK_PRIVATE_PARAMS with Finding("TR-SIG-001", Status.FAIL, ...) before signature verification, without echoing the secret value.
   • In check (plain trace format), checks _JWK_PRIVATE_PARAMS.intersection(jwk) instead of only "d" in jwk, emitting TR-SIG-004 (Status.FAIL) and TR-SIG-005 (Status.UNVERIFIED) for all seven private JWK parameters without echoing the secret value.

Cold-reader review (code-review sub-agent):

1. Finding 1 (src/agentrust_trace/provenance.py): The initial placement of the trusted_jwk private-key check ran after if revocation is not None: _check_not_revoked(trusted_jwk, revocation). When revocation was passed alongside a trusted_jwk containing private key material, _check_not_revoked ran first (invoking the external revocation store, or failing inside jwk_thumbprint for kty="oct").
   • Changed: Moved the trusted_jwk private-key check ahead of if revocation is not None: and added an assertion in test_provenance_verify_record_refuses_private_key_material_in_trusted_jwk verifying that a revocation store callback is never invoked when trusted_jwk carries private key material.
2. Finding 2 (conformance/tests/unit/test_tr_sig.py): test_plain_trace_all_private_jwk_members_fail_tr_sig_004 initially used an unsigned EC record at level=0; testing a validly signed OKP/Ed25519 plain trace record at level=1 directly pins that a validly signed record carrying any of the 7 private JWK members is never verified as TR-SIG-005 PASS.
   • Changed: Updated test_plain_trace_all_private_jwk_members_fail_tr_sig_004 to sign a valid OKP/Ed25519 plain trace record carrying each of the 7 private JWK members and check at level=1.

Contributor: Raghuvar Durga (@drk-ram)

Type of change

• Editorial (typo, link fix, clarification: no normative effect)
• Non-breaking spec change (new optional field, new platform profile, informative addition)
• Breaking spec change (requires 14-day comment period and Project Lead sign-off)
• Schema change
• Example addition

Spec section

• spec/server-provenance-v1.md / spec/server-provenance-v2.md reference implementation (src/agentrust_trace/provenance.py)
• spec/trace-v0.2.md section 3.2.1 conformance module (conformance/src/trace_tests/modules/tr_sig.py)

Tests for a fix

Verified with python tools/review_bar.py --base origin/main --head HEAD (19 fail on behaviour on origin/main, 0 fail on a missing name or file):

• tests/test_provenance_cnf_boundary.py::test_provenance_verify_record_refuses_private_key_material_in_cnf_jwk ([d], [p], [q], [dp], [dq], [qi], [k]): fails on origin/main with DID NOT RAISE ProvenanceError.
• tests/test_provenance_cnf_boundary.py::test_provenance_verify_record_refuses_private_key_material_in_trusted_jwk ([d], [p], [q], [dp], [dq], [qi], [k]): fails on origin/main with DID NOT RAISE ProvenanceError.
• tests/test_provenance_cnf_boundary.py::test_provenance_verify_record_wraps_pubkey_from_jwk_errors_in_provenance_error ([rsa], [ec], [x25519], [short-x], [padded-x]): fails on origin/main with unhandled ValueError.
• conformance/tests/unit/test_tr_sig.py::test_cmcp_runtime_private_key_material_in_cnf_jwk_fails ([d], [p], [q], [dp], [dq], [qi], [k]): fails when the check_cmcp_runtime private-key check is switched off (Status.PASS instead of Status.FAIL).
• conformance/tests/unit/test_tr_sig.py::test_plain_trace_all_private_jwk_members_fail_tr_sig_004 ([p], [q], [dp], [dq], [qi], [k]): fails when tr_sig.check only inspects "d" in jwk.

Validation commands run and results:

• pytest tests/test_provenance_cnf_boundary.py tests/test_provenance.py tests/test_public_functions_raise_what_they_document.py -q: passed (433 passed)
• pytest conformance/tests -q: passed (792 passed, 5 xpassed)
• python conformance/measurement/scripts/mutate_modules.py conformance: passed (27 checks measured, 27 verified, 0 unverified)
• python tools/review_bar.py --base origin/main --head HEAD: passed (19 fail on behaviour on base, 0 fail on missing name)
• ruff check src tests scripts: passed
• mypy src/agentrust_trace: passed
• python tools/check_dashes.py: passed

Checklist

• DCO sign-off on all commits (git commit -s)
• CHANGELOG.md updated (for any normative change)
• Breaking changes marked with <!-- CHANGED: #NNN: description --> in spec text
• Backward compatibility statement included (for breaking changes)

[rajnisht7]

This PR closes the three gaps from #535 cleanly. provenance.verify_record now rejects all seven private JWK members on both cnf.jwk and trusted_jwk, does that check before any revocation callback, and wraps _pubkey_from_jwk failures as ProvenanceError instead of leaking a bare ValueError. Conformance tr_sig matches: check_cmcp_runtime rejects the full private set early, and plain check no longer keys only on "d" it uses the same _JWK_PRIVATE_PARAMS intersection. The set matches models/tr_env/schema, messages name only member keys (tests assert the secret is never echoed), and the new tests cover all seven members on both surfaces, including the signed level-1 plain-trace case and the revocation-not-called assertion.

One small nit: when _pubkey_from_jwk(trusted_jwk) fails, the message still says “the record's embedded key is unusable”; something like “trusted_jwk is unusable” would be more accurate, but it is not a functional issue