Verification summary attestations communicate that an artifact has been verified at a specific SLSA level and details about that verification.

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 producers 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 resolvedDependencies of the artifact being verified (recursively) or b) matching the non-source dependency listed in resolvedDependencies (subject.digest == resolvedDependencies.digest and, ideally, vsa.resourceUri == resolvedDependencies.uri) to a VSA for that dependency and using vsa.verifiedLevels and vsa.dependencyLevels. Policy verifiers wishing to establish minimum requirements on dependencies SLSA levels may use vsa.dependencyLevels to do so.

Schema

// Standard attestation fields:
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
  "name": <NAME>,
  "digest": { <digest-in-request> }
}],

// Predicate
"predicateType": "https://slsa.dev/verification_summary/v1-rc2",
"predicate": {
  // Required
  "verifier": {
    "id": "<URI>"
  },
  "timeVerified": <TIMESTAMP>,
  "resourceUri": <artifact-URI-in-request>,
  "policy": {
    "uri": "<URI>",
    "digest": { /* DigestSet */ }
  }
  "inputAttestations": [
    {
      "uri": "<URI>",
      "digest": { <digest-of-attestation-data> }
    },
    ...
  ],
  "verificationResult": "<PASSED|FAILED>",
  "verifiedLevels": ["<SlsaResult>"],
  "dependencyLevels": {
    "<SlsaResult>": <Int>,
    "<SlsaResult>": <Int>,
    ...
  },
  "slsaVersion": "<MAJOR>.<MINOR>",
}

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.

timeVerified string (Timestamp), required

Timestamp indicating what time the verification occurred.

resourceUri string (ResourceURI), required

URI that identifies the resource associated with the artifact being verified.

policy object (ResourceDescriptor), required

Describes the policy that the subject was verified against.

The entry MUST contain a uri.

inputAttestations array (ResourceDescriptor), optional

The collection of attestations that were used to perform verification. Conceptually similar to the resolvedDependencies field in SLSA Provenance.

This field MAY be absent if the verifier does not support this feature. If non-empty, this field MUST contain information on all the attestations used to perform verification.

Each entry MUST contain a digest of the attestation and SHOULD contains a uri that can be used to fetch the attestation.

verificationResult string, required

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

verifiedLevels array (SlsaResult), required

Indicates the highest level of each track verified for the artifact (and not its dependencies), or “FAILED” if policy verification failed.

Users MUST NOT include more than one level per SLSA track. Note that each SLSA level implies all levels below it (e.g. SLSA_BUILD_LEVEL_3 implies SLSA_BUILD_LEVEL_2 and SLSA_BUILD_LEVEL_1), so there is no need to include more than one level per track.

dependencyLevels object, optional

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.

Users MUST count each dependency only once per SLSA track, at the highest level verified. For example, if a dependency meets SLSA_BUILD_LEVEL_2, you include it with the count for SLSA_BUILD_LEVEL_2 but not the count for SLSA_BUILD_LEVEL_1.

slsaVersion string, optional

Indicates the version of the SLSA specification that the verifier used, in the form <MAJOR>.<MINOR>. Example: 1.0. If unset, the default is an unspecified minor version of 1.x.

Example

WARNING: This is just for demonstration purposes.

"_type": "https://in-toto.io/Statement/v1",
"subject": [{
  "name": "out/example-1.2.3.tar.gz",
  "digest": {"sha256": "5678..."}
}],

// Predicate
"predicateType": "https://slsa.dev/verification_summary/v1-rc2",
"predicate": {
  "verifier": {
    "id": "https://example.com/publication_verifier"
  },
  "timeVerified": "1985-04-12T23:20:50.52Z",
  "resourceUri": "https://example.com/example-1.2.3.tar.gz",
  "policy": {
    "uri": "https://example.com/example_tarball.policy",
    "digest": {"sha256": "1234..."}
  },
  "inputAttestations": [
    {
      "uri": "https://example.com/provenances/example-1.2.3.tar.gz.intoto.jsonl",
      "digest": {"sha256": "abcd..."}
    }
  ],
  "verificationResult": "PASSED",
  "verifiedLevels": ["SLSA_LEVEL_3"],
  "dependencyLevels": {
    "SLSA_BUILD_LEVEL_3": 5,
    "SLSA_BUILD_LEVEL_2": 7,
    "SLSA_BUILD_LEVEL_1": 1,
  },
  "slsaVersion": "1.0"
}

SlsaResult (String)

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

  • SLSA_BUILD_LEVEL_0
  • SLSA_BUILD_LEVEL_1
  • SLSA_BUILD_LEVEL_2
  • SLSA_BUILD_LEVEL_3
  • FAILED (Indicates policy evaluation failed)

Note that each SLSA level implies the levels below it. For example, SLSA_BUILD_LEVEL_3 means (SLSA_BUILD_LEVEL_1 + SLSA_BUILD_LEVEL_2 + SLSA_BUILD_LEVEL_3).

Users MAY use custom values here but MUST NOT use custom values starting with SLSA_.

Change history

  • 1:
    • Replaced materials with resolvedDependencies.
    • Relaxed SlsaResult to allow other values.
    • Converted to lowerCamelCase for consistency with SLSA Provenance.
    • Added slsaVersion field.
  • 0.2:
    • Added resource_uri field.
    • Added optional input_attestations field.
  • 0.1: Initial version.