[drk-ram]

What this changes

Closes #506.

Adds an executable, synthetic example under examples/signature-execution-boundary/ and automated checks in tests/test_signature_execution_boundary.py demonstrating the boundary between a valid Ed25519 signature and verified execution in TRACE v0.2, building on docs/quickstart.md, docs/tutorials/verifying-a-trust-record.md, and LIMITATIONS.md.

Run python examples/signature-execution-boundary/signature_execution_boundary.py --out DIR to generate deterministic UTF-8/LF JSON artifacts (baseline-record.trace.json, invented-execution-record.trace.json, and verification-inputs.json) and execute four boundary checks:

1. Valid signature against a separately retained trusted key: verify_record() validates baseline-record.trace.json against schema/trace-claim.json, confirms eat_profile, matches cnf.jwk to the caller's configured trusted_key thumbprint (8WEkIEIu0xGWVjA1V8weTlFmpl0D7N3EHC0yfRjfnQQ), checks freshness at now=1791936000, and returns VerificationResult with revocation.outcome == "no_check_performed" and platform_measurement.outcome == "not_attempted".
2. Post-signing field mutation rejected: Modifying a signed field after signing (data_class in the script, and parametrized across subject, iat, data_class, model.model_id, runtime.measurement, policy.bundle_hash, policy.enforcement_mode, build_provenance.digest, appraisal.status, tool_transcript.hash, and tool_transcript.call_count in tests/test_signature_execution_boundary.py) still passes validate_json() but raises cryptography.exceptions.InvalidSignature.
3. Wrong issuer key and cnf.jwk confirmation-key binding: Verifying against wrong_trusted_key raises ValueError (cnf.jwk thumbprint mismatch); replacing record["cnf"]["jwk"] with wrong_trusted_key without re-signing passes validate_json() and raises InvalidSignature because cnf.jwk is part of the RFC 8785 preimage; by contrast, allow_embedded_key=True emits a UserWarning and proves internal self-consistency only.
4. Invented execution claims signed by the trusted issuer: invented-execution-record.trace.json, signed by the same trusted issuer key at the same (subject, iat) with fabricated claims (policy.enforcement_mode: "enforce", build_provenance.slsa_level: 3, appraisal.status: "affirming", and tool_transcript.call_count: 12), passes validate_json() and verify_record() with an identical VerificationResult. Only independent comparison of tool_transcript.hash against the retained external transcript separates baseline=matched from invented=mismatch and unretained=unavailable.

No specification, schema, or SDK changes.

Cold-reader review (CONTRIBUTING.md)

An independent cold-reader session reviewed the initial diff (c08974c3c0b33d9bd77bc1778b238a86f770b788..71f02fb) against CONTRIBUTING.md, docs/quickstart.md, docs/tutorials/verifying-a-trust-record.md, LIMITATIONS.md, and examples/mcp-retry/:

• Script vs. test parity (README.md Checks 2, 3, 4): The initial signature_execution_boundary.py did not exercise allow_embedded_key=True (UserWarning) in Check 3 or None -> "unavailable" in Check 4 (both were only in tests/test_signature_execution_boundary.py), and README.md Check 2 omitted model from the list of signed claims. Updated signature_execution_boundary.py to add commitment_status() and execute both checks directly, and clarified the README.md table.
• Canonicalization domain traversal (tuple and non-finite floats): canonical_bytes() and _commitment() initially traversed dict and list but not tuple, and test_example_numeric_domain_refuses_floats_and_unsafe_integers omitted "NaN", "Infinity", and "-Infinity". Extended both walkers to (list, tuple) and added parametrized tests for "NaN", "Infinity", "-Infinity", and tuple-nested floats/unsafe integers.
• README stdout drift guard: Added an assertion in test_script_reproduces_fixed_evidence_bytes_and_stdout that extracts the text output block from examples/signature-execution-boundary/README.md and compares it byte-for-byte against the script's stdout.

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

Informative example and test suite for §3.2.2 (signature and key binding) and §3.3 (verification vs. appraisal); no normative text modified.

Tests for a fix

Not a code or schema fix.

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)

[imran-siddique]

Verified locally: the script prints 4 of 4 as documented and the 49 tests pass. The README's pinned commit c08974c3 predates the example; the CHANGELOG follow-up PR repins it to the merge commit.