Experimental RFC9162_SHA256 receipt support for IETF 126

[Amaury Chamayou (achamayou)]

Context

This is an experimental addition for interoperability work at the IETF 126 Hackathon. It adds early pyscitt support for RFC 9942 receipts using the RFC9162_SHA256 profile (vds=1). It is intentionally scoped to the published examples rather than presented as complete RFC 9942 profile coverage.

The existing CCF receipt path remains separate and is dispatched only for vds=2.

What changed

• Reconstruct RFC 9162 SHA-256 Merkle roots from RFC 9942 inclusion proofs and verify detached COSE signatures with pycose.
• Dispatch receipts from the protected vds header: RFC9162_SHA256 for vds=1, CCF for vds=2, and reject missing, malformed, or unknown values.
• Keep validation centered on Transparent Statements while allowing an explicit --service-key as an alternative to a service trust store.
• Pin the IETF SCITT valid-es256 and fail-tampered-path vectors at commit 727ee03d86fa2c2ca8c534584b870235a1b252df.
• Add derived Transparent Statements that embed each published receipt under header label 394 and strip byte-for-byte to the original signed statements.
• Verify the valid example and reject the published tampered inclusion path with a clear proof/signature error.

Experimental notes

• The published examples do not carry a kid, so validation uses an explicitly trusted log key.
• The source vectors provide receipts separately; the checked-in Transparent Statements are derived test inputs for pyscitt's unified validation route.

Validation

• pytest test/test_rfc9942.py test/test_verify.py -q — 28 passed
• CI-equivalent mypy, isort, Black, C/C++ formatting, notice, and changelog checks passed locally
• The complete CCF-backed functional suite is left to the configured upstream CI runner

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Pull request overview

Adds experimental pyscitt verification support for RFC 9942 Transparent Statements carrying RFC9162_SHA256 receipts (vds=1), while keeping the existing CCF receipt verification path for vds=2.

Changes:

• Implement RFC 9162 SHA-256 Merkle root reconstruction from RFC 9942 inclusion proofs and verify detached COSE signatures via pycose.
• Dispatch receipt verification based on protected header vds (accept 1 and 2; reject missing/malformed/unknown).
• Add CLI support for explicitly trusting a PEM service public key (--service-key) and add pinned IETF example vectors + tests.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pyscitt/pyscitt/verify.py	Adds vds-based dispatch and RFC9162_SHA256 receipt verification implementation.
pyscitt/pyscitt/cli/validate.py	Adds --service-key trust option and wiring into transparent statement validation.
pyscitt/README.md	Documents validating RFC 9942 Transparent Statements with --service-key.
test/test_rfc9942.py	Adds tests covering pinned vectors, transparent statement stripping, dispatch, and tampered-path rejection.
test/test_verify.py	Updates transparent statement verification tests to set vds=2 on synthetic CCF receipts.
.gitattributes	Marks .cose vectors as binary and disables text normalization for .pub keys.
test/test_vectors/scitt-cose/v1/valid-es256/transparent-statement.cose	Adds pinned “valid-es256” transparent statement test vector.
test/test_vectors/scitt-cose/v1/valid-es256/statement.cose	Adds pinned “valid-es256” statement test vector.
test/test_vectors/scitt-cose/v1/valid-es256/receipt.cose	Adds pinned “valid-es256” receipt test vector.
test/test_vectors/scitt-cose/v1/valid-es256/log-key.pub	Adds pinned “valid-es256” log public key.
test/test_vectors/scitt-cose/v1/fail-tampered-path/transparent-statement.cose	Adds pinned “fail-tampered-path” transparent statement test vector.
test/test_vectors/scitt-cose/v1/fail-tampered-path/statement.cose	Adds pinned “fail-tampered-path” statement test vector.
test/test_vectors/scitt-cose/v1/fail-tampered-path/receipt.cose	Adds pinned “fail-tampered-path” receipt test vector.
test/test_vectors/scitt-cose/v1/fail-tampered-path/log-key.pub	Adds pinned “fail-tampered-path” log public key.

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[Amaury Chamayou (achamayou)]

This is allowing a service key to be passed directly, instead of a path to a trust store containing certificates. That's necessary to support baseline COSE Receipts with RFC9162_SHA256 proofs and no x509 integration.

[Amaury Chamayou (achamayou)]

The main change is to dispatch on VDS type here. The CCF branch is unchanged of course.

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