Note about 0.x versions

We expect regular iteration on 0.x versions until 1.0. During 0.x, we lean towards smaller, faster releases in order to get earlier feedback on the design. After 1.0, we will limit the frequency of breaking changes.

To make this manageable, we recommend that:

  • Generators choose the latest 0.x version at the time of implementation and then stick with that until 1.0 unless there is reason to upgrade before then.
  • Consumers accept all known versions and convert internally between them.

Purpose

Describe what SLSA level an artifact or set of artifacts was verified at and other details about the verification process including what SLSA level the dependencies were verified at.

This allows software consumers to make a decision about the validity of an artifact without needing to have access to all of the attestations about the artifact or all of its transitive dependencies. They can use it to delegate complex policy decisions to some trusted party and then simply trust that party’s decision regarding the artifact.

It also allows software publishers to keep the details of their build pipeline confidential while still communicating that some verification has taken place. This might be necessary for legal reasons (keeping a software supplier confidential) or for security reasons (not revealing that an embargoed patch has been included).

Prerequisite

Understanding of SLSA Software Attestations, SLSA Provenance, and the larger in-toto attestation framework.

Model

A Verification Summary Attestation (VSA) is an attestation that some entity (verifier) verified one or more software artifacts (the subject of an in-toto attestation Statement) by evaluating the artifact and a bundle of attestations against some policy. Users who trust the verifier may assume that the artifacts met the indicated SLSA level without themselves needing to evaluate the artifact or to have access to the attestations the verifier used to make its determination.

The VSA also allows consumers to determine the verified levels of all of an artifact’s transitive dependencies. The verifier does this by either a) verifying the provenance of each non-source dependency listed in the materials of the artifact being verified (recursively) or b) matching the non-source dependency listed in materials (by subject.digest == materials.digest and, ideally, subject.name == materials.uri) to a VSA for that dependency and using vsa.policy_level and vsa.dependency_levels. Policy verifiers wishing to establish minimum requirements on dependencies SLSA levels may use vsa.dependency_levels to do so.

Schema

// Standard attestation fields:
"_type": "https://in-toto.io/Statement/v0.1",
"subject": [{
  "name": <artifact-URI-in-request>,
  "digest": { <digest-in-request> }
}],

// Predicate
"predicateType": "https://slsa.dev/verification_summary/v0.1",
"predicate": {
  // Required
  "verifier": {
    "id": "<URI>"
  },
  "time_verified": <TIMESTAMP>,
  "policy": {
    "uri": "<URI>",
    "digest": { /* DigestSet */ }
  }
  "verification_result": "<PASSED|FAILED>",
  "policy_level": "<SlsaResult>",
  "dependency_levels": {
    "<SlsaResult>": <Int>,
    "<SlsaResult>": <Int>,
    ...
  }
}

Parsing rules

This predicate follows the in-toto attestation parsing rules. Summary:

  • Consumers MUST ignore unrecognized fields.
  • The predicateType URI includes the major version number and will always change whenever there is a backwards incompatible change.
  • Minor version changes are always backwards compatible and “monotonic.” Such changes do not update the predicateType.
  • Producers MAY add extension fields using field names that are URIs.

Fields

NOTE: This section describes the fields within predicate. For a description of the other top-level fields, such as subject, see Statement.

verifier object, required

Identifies the entity that performed the verification.

The identity MUST reflect the trust base that consumers care about. How detailed to be is a judgment call.

Consumers MUST accept only specific (signer, verifier) pairs. For example, “GitHub” can sign provenance for the “GitHub Actions” verifier, and “Google” can sign provenance for the “Google Cloud Deploy” verifier, but “GitHub” cannot sign for the “Google Cloud Deploy” verifier.

The field is required, even if it is implicit from the signer, to aid readability and debugging. It is an object to allow additional fields in the future, in case one URI is not sufficient.

verifier.id string (TypeURI), required

URI indicating the verifier’s identity.

time_verified string (Timestamp), required

Timestamp indicating what time the verification occurred.

policy object, required

Describes the policy that was used to verify this artifact.

policy.uri string (ResourceURI), required

The URI of the policy used to perform verification.

policy.digest object (DigestSet), optional

Collection of cryptographic digests for the contents of the policy used to perform verification.

verification_result string, required

Either “PASSED” or “FAILED” to indicate if the artifact passed or failed the policy verification.

policy_level string (SlsaResult), required

Indicates what SLSA level the artifact itself (and not its dependencies) was verified at or “FAILED” if policy verification failed.

dependency_levels object, required

A count of the dependencies at each SLSA level.

Map from SlsaResult to the number of the artifact’s transitive dependencies that were verified at the indicated level. Absence of a given level of SlsaResult MUST be interpreted as reporting 0 dependencies at that level.

Example

WARNING: This is just for demonstration purposes.

"_type": "https://in-toto.io/Statement/v0.1",
"subject": [{
  "name": "https://example.com/example-1.2.3.tar.gz",
  "digest": {"sha256": "5678..."}
}],

// Predicate
"predicateType": "https://slsa.dev/verification_summary/v0.1",
"predicate": {
  "verifier": {
    "id": "https://example.com/publication_verifier"
  },
  "time_verified": "1985-04-12T23:20:50.52Z",
  "policy": {
    "uri": "https://example.com/example_tarball.policy",
    "digest": {"sha256": "1234..."}
  },
  "verification_result": "PASSED",
  "policy_level": "SLSA_LEVEL_3",
  "dependency_levels": {
    "SLSA_LEVEL_4": 1,
    "SLSA_LEVEL_3": 5,
    "SLSA_LEVEL_2": 7,
    "SLSA_LEVEL_1": 1,
  }
}

SlsaResult (String)

The result of evaluating an artifact (or set of artifacts) against SLSA. Must be one of these values:

  • SLSA_LEVEL_0
  • SLSA_LEVEL_1
  • SLSA_LEVEL_2
  • SLSA_LEVEL_3
  • SLSA_LEVEL_4
  • FAILED (Indicates policy evaluation failed)

Change history

  • 0.1: Initial version.