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

This document defines the following predicate type within the in-toto attestation framework:

"predicateType": "https://slsa.dev/verification_summary/v1"

Important: Always use the above string for predicateType rather than what is in the URL bar. The predicateType URI will always resolve to the latest minor version of this specification. See parsing rules for more information.

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).

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",
"predicate": {
  "verifier": {
    "id": "<URI>",
    "version": {
      "<COMPONENT>": "<VERSION>",
      ...
    }
  },
  "timeVerified": <TIMESTAMP>,
  "resourceUri": <artifact-URI-in-request>,
  "policy": {
    "uri": "<URI>",
    "digest": { <digest-of-policy-data> }
  }
  "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.

verifier.version map (string->string), optional

Map of names of components of the verification platform to their version.

timeVerified string (Timestamp), optional

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 identifying which policy was applied and SHOULD contain a digest to indicate the exact version of that policy.

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. A set but empty dependencyLevels object means that the artifact has no dependency at all, while an unset or null dependencyLevels means that the verifier makes no claims about the artifact’s dependencies.

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",
"predicate": {
  "verifier": {
    "id": "https://example.com/publication_verifier",
    "version": {
      "slsa-verifier-linux-amd64": "v2.3.0",
      "slsa-framework/slsa-verifier/actions/installer": "v2.3.0"
    }
  },
  "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_BUILD_LEVEL_3"],
  "dependencyLevels": {
    "SLSA_BUILD_LEVEL_3": 5,
    "SLSA_BUILD_LEVEL_2": 7,
    "SLSA_BUILD_LEVEL_1": 1,
  },
  "slsaVersion": "1.0"
}

How to verify

VSA consumers use VSAs to accomplish goals based on delegated trust. We call the process of establishing a VSA’s authenticity and determining whether it meets the consumer’s goals ‘verification’. Goals differ, as do levels of confidence in VSA producers, so the verification procedure changes to suit its context. However, there are certain steps that most verification procedures have in common.

Verification MUST include the following steps:

  1. Verify the signature on the VSA envelope using the preconfigured roots of trust. This step ensures that the VSA was produced by a trusted producer and that it hasn’t been tampered with.

  2. Verify the statement’s subject matches the digest of the artifact in question. This step ensures that the VSA pertains to the intended artifact.

  3. Verify that the predicateType is https://slsa.dev/verification_summary/v1. This step ensures that the in-toto predicate is using this version of the VSA format.

  4. Verify that the verifier matches the public key (or equivalent) used to verify the signature in step 1. This step identifies the VSA producer in cases where their identity is not implicitly revealed in step 1.

  5. Verify that the value for resourceUri in the VSA matches the expected value. This step ensures that the consumer is using the VSA for the producer’s intended purpose.

  6. Verify that the value for slsaResult is PASSED. This step ensures the artifact is suitable for the consumer’s purposes.

  7. Verify that verifiedLevels contains the expected value. This step ensures that the artifact is suitable for the consumer’s purposes.

Verification MAY additionally contain the following step:

  1. (Optional) Verify additional fields required to determine whether the VSA meets your goal.

Verification mitigates different threats depending on the VSA’s contents and the verification procudure.

IMPORTANT: A VSA does not protect against compromise of the verifier, such as by a malicious insider. Instead, VSA consumers SHOULD carefully consider which verifiers they add to their roots of trust.

Examples

  1. Suppose consumer C wants to delegate to verifier V the decision for whether to accept artifact A as resource R. Consumer C verifies that:

    • The signature on the VSA envelope using V’s public signing key from their preconfigured root of trust.

    • subject is A.

    • predicateType is https://slsa.dev/verification_summary/v1.

    • verifier.id is V.

    • resourceUri is R.

    • slsaResult is PASSED.

    • verifiedLevels contains SLSA_BUILD_LEVEL_UNEVALUATED.

    Note: This example is analogous to traditional code signing. The expected value for verifiedLevels is arbitrary but prenegotiated by the producer and the consumer. The consumer does not need to check additional fields, as C fully delegates the decision to V.

  2. Suppose consumer C wants to enforce the rule “Artifact A at resource R must have a passing VSA from verifier V showing it meets SLSA Build Level 2+.” Consumer C verifies that:

    • The signature on the VSA envelope using V’s public signing key from their preconfigured root of trust.

    • subject is A.

    • predicateType is https://slsa.dev/verification_summary/v1.

    • verifier.id is V.

    • resourceUri is R.

    • slsaResult is PASSED.

    • verifiedLevels is SLSA_BUILD_LEVEL_2 or SLSA_BUILD_LEVEL_3.

    Note: In this example, verifying the VSA mitigates the same threats as verifying the artifact’s SLSA provenance. See Verifying artifacts for details about which threats are addressed by verifying each SLSA level.

SlsaResult (String)

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

  • SLSA_BUILD_LEVEL_UNEVALUATED
  • 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 in the same track. 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

  • 1.1:
    • Changed the policy object to recommend that the digest field of the ResourceDescriptor is set.
    • Added optional verifier.version field to record verification tools.
    • Added Verification section with examples.
    • Made timeVerified optional.
  • 1.0:
    • 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.
‹ Provenance