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OSCAL Signing Predicate v1¤

Trestle uses this predicate type in the in-toto Statement created by trestle sign:

https://oscal-compass.github.io/compliance-trestle/predicates/oscal-signing/v1

The predicate describes how trestle created the digest for a signed JSON artifact. The predicate type URI is recorded in the signed Statement and is not fetched during verification.

For command usage, see the trestle sign and trestle verify CLI documentation.

What is signed¤

trestle sign creates a detached DSSE envelope. The envelope payload is an in-toto Statement that records:

  • the subject file name
  • the selected digest of the RFC 8785 canonical JSON bytes (SHA-256 by default)
  • the predicate type and predicate fields described below

The DSSE signature is made over the DSSE pre-authentication encoding of the Statement payload. The original JSON file is not modified.

Statement¤

The DSSE payload is a base64-encoded in-toto Statement with exactly one subject. This example uses the default SHA-256 algorithm:

{
  "_type": "https://in-toto.io/Statement/v1",
  "subject": [
    {
      "name": "catalog.json",
      "digest": {
        "sha256": "..."
      }
    }
  ],
  "predicateType": "https://oscal-compass.github.io/compliance-trestle/predicates/oscal-signing/v1",
  "predicate": {
    "canonicalization": "RFC8785",
    "digestAlgorithm": "sha256",
    "digestSource": "canonical-json",
    "tool": "compliance-trestle"
  }
}

Predicate fields¤

Field Description
canonicalization The canonicalization algorithm applied before hashing. Trestle uses RFC8785.
digestAlgorithm The selected digest algorithm in in-toto notation, such as sha256, sha384, or sha3_256. Defaults to sha256.
digestSource The bytes that were hashed. canonical-json means the digest was computed over the RFC 8785 canonical JSON bytes, not the original file bytes.
tool The producer of the predicate. Trestle writes compliance-trestle.

Supported digest names are sha224, sha256, sha384, sha512, sha3_224, sha3_256, sha3_384, and sha3_512. After verifying the DSSE signature, verification detects the algorithm from the authenticated predicate's digestAlgorithm and checks the corresponding subject digest. An explicit --digest-algorithm additionally requires the predicate to identify that algorithm. The algorithm metadata is covered by the DSSE signature.

See Digest algorithms for CLI selection and compatibility with existing SHA-256 envelopes.