Provenance

To trace software back to the source and define the moving parts in a complex supply chain, provenance needs to be there from the very beginning. It’s the verifiable information about software artifacts describing where, when and how something was produced. For higher SLSA levels and more resilient integrity guarantees, provenance requirements are stricter and need a deeper, more technical understanding of the predicate.

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

"predicateType": "https://slsa.dev/provenance/v1-rc1"

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.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.

Purpose

Describe how an artifact or set of artifacts was produced so that:

• Consumers of the provenance can verify that the artifact was built according to expectations.
• Others can rebuild the artifact, if desired.

This predicate is the RECOMMENDED way to satisfy the SLSA v1.0 provenance requirements.

Model

Provenance is an attestation that the builder produced the subject software artifacts through execution of the buildDefinition.

The model is as follows:

• Each build runs as an independent process on a multi-tenant platform. The builder.id identifies this platform, representing the transitive closure of all entities that are trusted to faithfully run the build and record the provenance. (Note: The same model can be used for platform-less or single-tenant build systems.)

• The build process is defined by a parameterized template, identified by buildType. This encapsulates the process that ran, regardless of what system ran it. Often the build type is specific to the build platform because most build platforms have their own unique interfaces.

• All top-level, independent inputs are captured by the parameters to the template. There are two types of parameters:

  • externalParameters: the external interface to the build. In SLSA, these values are untrusted; they MUST be included in the provenance and MUST be verified downstream.

  • systemParameters: set internally by the platform. In SLSA, these values are trusted because the platform is trusted; they are OPTIONAL and need not be verified downstream. They MAY be included to enable reproducible builds, debugging, or incident response.

• All artifacts fetched during initialization or execution of the build process are considered dependencies, including those referenced directly by parameters. The resolvedDependencies captures these dependencies, if known. For example, a build that takes a git repository URI as a parameter might record the specific git commit that the URI resolved to as a dependency.

• During execution, the build process might communicate with the build platform’s control plane and/or build caches. This communication is not captured directly in the provenance, but is instead implied by builder.id and subject to SLSA Requirements. Such communication SHOULD NOT influence the definition of the build; if it does, it SHOULD go in resolvedDependencies instead.

• Finally, the build process outputs one or more artifacts, identified by subject.

For concrete examples, see index of build types.

Parsing rules

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

• Consumers MUST ignore unrecognized fields unless otherwise noted.
• 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.
• Unset, null, and empty field values MUST be interpreted equivalently.

Schema

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

{
    // Standard attestation fields:
    "_type": "https://in-toto.io/Statement/v1",
    "subject": [...],

    // Predicate:
    "predicateType": "https://slsa.dev/provenance/v1-rc1",
    "predicate": {
        "buildDefinition": {
            "buildType": string,
            "externalParameters": object,
            "systemParameters": object,
            "resolvedDependencies": [ ...#ArtifactReference ],
        },
        "runDetails": {
            "builder": {
                "id": string,
                "version": string,
                "builderDependencies": [ ...#ArtifactReference ],
            },
            "metadata": {
                "invocationId": string,
                "startedOn": #Timestamp,
                "finishedOn": #Timestamp,
            },
            "byproducts": [ ...#ArtifactReference ],
        }
    }
}

#ArtifactReference: {
    "uri": string,
    "digest": {
        "sha256": string,
        "sha512": string,
        "sha1": string,
        // TODO: list the other standard algorithms
        [string]: string,
    },
    "localName": string,
    "downloadLocation": string,
    "mediaType": string,
}

#Timestamp: string  // <YYYY>-<MM>-<DD>T<hh>:<mm>:<ss>Z

syntax = "proto3";

package slsa.v1;

import "google/protobuf/struct.proto";
import "google/protobuf/timestamp.proto";

// NOTE: While file uses snake_case as per the Protocol Buffers Style Guide, the
// provenance is always serialized using JSON with lowerCamelCase. Protobuf
// tooling performs this case conversion automatically.

message Provenance {
  BuildDefinition build_definition = 1;
  RunDetails run_details = 2;
}

message BuildDefinition {
  string build_type = 1;
  google.protobuf.Struct external_parameters = 2;
  google.protobuf.Struct system_parameters = 3;
  repeated ArtifactReference resolved_dependencies = 4;
}

message ArtifactReference {
  string uri = 1;
  map<string, string> digest = 2;
  string local_name = 3;
  string download_location = 4;
  string media_type = 5;
}

message RunDetails {
  Builder builder = 1;
  BuildMetadata metadata = 2;
  repeated ArtifactReference byproducts = 3;
}

message Builder {
  string id = 1;
  map<string, string> version = 2;
  repeated ArtifactReference builder_dependencies = 3;
}

message BuildMetadata {
  string invocation_id = 1;
  google.protobuf.Timestamp started_on = 2;
  google.protobuf.Timestamp finished_on = 3;
}

Provenance

REQUIRED for SLSA Build L1: buildDefinition, runDetails

BuildDefinition

REQUIRED for SLSA Build L1: buildType, externalParameters

The BuildDefinition describes all of the inputs to the build. It SHOULD contain all the information necessary and sufficient to initialize the build and begin execution.

The externalParameters and systemParameters are the top-level inputs to the template, meaning inputs not derived from another input. Each is an arbitrary JSON object, though it is RECOMMENDED to keep the structure simple with string values to aid verification. The same field name SHOULD NOT be used for both externalParameters and systemParameters.

The parameters SHOULD only contain the actual values passed in through the interface to the build system. Metadata about those parameter values, particularly digests of artifacts referenced by those parameters, SHOULD instead go in resolvedDependencies. The documentation for buildType SHOULD explain how to convert from a parameter to the dependency uri. For example:

"externalParameters": {
    "repository": "https://github.com/octocat/hello-world",
    "ref": "refs/heads/main"
},
"resolvedDependencies": [{
    "uri": "git+https://github.com/octocat/hello-world@refs/heads/main",
    "digest": {"sha1": "7fd1a60b01f91b314f59955a4e4d4e80d8edf11d"}
}]

Guidelines:

• Maximize the amount of information that is implicit from the meaning of buildType. In particular, any value that is boilerplate and the same for every build SHOULD be implicit.

• Reduce parameters by moving configuration to input artifacts whenever possible. For example, instead of passing in compiler flags via an external parameter that has to be verified separately, require the flags to live next to the source code or build configuration so that verifying the latter automatically verifies the compiler flags.

• In some cases, additional external parameters might exist that do not impact the behavior of the build, such as a deadline or priority. These extra parameters SHOULD be excluded from the provenance after careful analysis that they indeed pose no security impact.

• If possible, architect the build system to use this definition as its sole top-level input, in order to guarantee that the information is sufficient to run the build.

• When build configuration is evaluated client-side before being sent to the server, such as transforming version-controlled YAML into ephemeral JSON, some solution is needed to make verification practical. Consumers need a way to know what configuration is expected and the usual way to do that is to map it back to version control, but that is not possible if the server cannot verify the configuration’s origins. Possible solutions:

  • (RECOMMENDED) Rearchitect the build service to read configuration directly from version control, recording the server-verified URI in externalParameters and the digest in resolvedDependencies.

  • Record the digest in the provenance¹ and use a separate provenance attestation to link that digest back to version control. In this solution, the client-side evaluation is considered a separate “build” that SHOULD be independently secured using SLSA, though securing it can be difficult since it usually runs on an untrusted workstation.

• The purpose of resolvedDependencies is to facilitate recursive analysis of the software supply chain. Where practical, it is valuable to record the URI and digest of artifacts that, if compromised, could impact the build. At SLSA Build L3, completeness is considered “best effort”.

⚠ RFC: We are particularly looking for feedback on this schema from potential implementers. Does this model map cleanly to existing build systems? Is it natural to identify and express the external parameters? Is anything confusing or ambiguous?

ArtifactReference

REQUIRED: at least one of uri or digest

Example:

{
    "uri": "pkg:pypi/pyyaml@6.0",
    "digest": {"sha256": "5f0689d54944564971f2811f9788218bfafb21aa20f532e6490004377dfa648f"},
    "localName": "PyYAML-6.0.tar.gz",
    "downloadLocation": "https://files.pythonhosted.org/packages/36/2b/61d51a2c4f25ef062ae3f74576b01638bebad5e045f747ff12643df63844/PyYAML-6.0.tar.gz",
    "mediaType": "application/gzip"
}

⚠ RFC: Do we need all these fields? Is this adding too much complexity?

RunDetails

REQUIRED for SLSA Build L1: builder

Builder

REQUIRED for SLSA Build L1: id

The builder represents the transitive closure of all the entities that are, by necessity, trusted to faithfully run the build and record the provenance.

The id MUST reflect the trust base that consumers care about. How detailed to be is a judgement call. For example, GitHub Actions supports both GitHub-hosted runners and self-hosted runners. The GitHub-hosted runner might be a single identity because it’s all GitHub from the consumer’s perspective. Meanwhile, each self-hosted runner might have its own identity because not all runners are trusted by all consumers.

Consumers MUST accept only specific signer-builder pairs. For example, “GitHub” can sign provenance for the “GitHub Actions” builder, and “Google” can sign provenance for the “Google Cloud Build” builder, but “GitHub” cannot sign for the “Google Cloud Build” builder.

Design rationale: The builder is distinct from the signer in order to support the case where one signer generates attestations for more than one builder, as in the GitHub Actions example above. 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.

⚠ RFC: Do we need more explicit guidance on how to choose a URI?

⚠ RFC: Would it be preferable to allow builders to set arbitrary properties, rather than calling out version and builderDependencies? We don’t expect verifiers to use any of them, so maybe that’s the simpler approach? Or have a properties that is an arbitrary object? (#319)

⚠ RFC: Do we want/need to identify the tenant of the build system, separately from the build system itself? If so, a single id that combines both (e.g. https://builder.example/tenants/company1.example/project1) or two separate fields (e.g. {"id": "https://builder.example", "tenant": "https://company1.example/project1"})? What would the use case be for this? How does verification work?

BuildMetadata

REQUIRED: (none)

Verification

Verification of provenance encompasses the following steps.

Step 1: Check SLSA Build level

First, check the SLSA Build level by comparing the artifact to its provenance and the provenance to a preconfigured root of trust. The goal is to ensure that the provenance actually applies to the artifact in question and to assess the trustworthiness of the provenance. This mitigates some or all of threats “D”, “F”, “G”, and “H”, depending on SLSA Build level and where verification happens.

Up front:

• Configure the verifier’s roots of trust, meaning the recognized builder identities and the maximum SLSA Build level each builder is trusted up to. Different verifiers might use different roots of trust, but usually a verifier uses the same roots of trust for all packages. This is likely in the form of a map from (builder public key identity, builder.id) to (SLSA Build level).

  Example root of trust configuration

  The following snippet shows conceptually how a verifier’s roots of trust might be configured using made-up syntax.

  "slsaRootsOfTrust": [
      // A builder trusted at SLSA Build L3, using a fixed public key.
      {
          "publicKey": "HKJEwI...",
          "builderId": "https://somebuilder.example.com/slsa/l3",
          "slsaBuildLevel": 3
      },
      // A different builder that claims to be SLSA Build L3,
      // but this verifier only trusts it to L2.
      {
          "publicKey": "tLykq9...",
          "builderId": "https://differentbuilder.example.com/slsa/l3",
          "slsaBuildLevel": 2
      },
      // A builder that uses Sigstore for authentication, without a builder.id
      {
          "sigstore": {
              "root": "global",  // identifies fulcio/rekor roots
              "subjectAlternativeNamePattern": "https://github.com/slsa-framework/slsa-github-generator/.github/workflows/generator_generic_slsa3.yml@refs/tags/v*.*.*"
          }
          "builderId": "",  // empty for this particular builder
          "slsaBuildLevel": 3,
      }
      ...
  ],
  

Given an artifact and its provenance:

1. Verify the envelope’s signature using the roots of trust, resulting in a list of recognized public keys (or equivalent).
2. Verify that statement’s subject matches the digest of the artifact in question.
3. Verify that the predicateType is https://slsa.dev/provenance/v1-rc1.
4. Look up the SLSA Build Level in the roots of trust, using the recognized public keys and the builder.id, defaulting to SLSA Build L1.

Resulting threat mitigation:

• Threat “D”: SLSA Build L3 requires protection against compromise of the build process and provenance generation by an external adversary, such as persistence between builds or theft of the provenance signing key. In other words, SLSA Build L3 establishes that the provenance is accurate and trustworthy, assuming you trust the build platform.
  • IMPORTANT: SLSA Build L3 does not cover compromise of the build platform itself, such as by a malicious insider. Instead, verifiers SHOULD carefully consider which build platforms are added to the roots of trust. For advice on establishing trust in build platforms, see Verifying build systems.
• Threat “F”: SLSA Build L2 covers tampering of the artifact or provenance after the build. This is accomplished by verifying the subject and signature in the steps above.
• Threat “G”: Verification by the consumer or otherwise outside of the package registry covers compromise of the registry itself. (Verifying within the registry at publication time is also valuable, but does not cover Threat “G” or “H”.)
• Threat “H”: Verification by the consumer covers compromise of the package in transit. (Many ecosystems also address this threat using package signatures or checksums.)
  • NOTE: SLSA does not cover adversaries tricking a consumer to use an unintended package, such as through typosquatting.

Step 2: Check expectations

Next, check that the package’s provenance meets expectations for that package in order to mitigate threat “C”.

In our threat model, the adversary has ability to invoke a build and to publish to the registry but not to write to the source repository, nor do they have insider access to any trusted systems. Expectations MUST be sufficient to detect or prevent this adversary from injecting unofficial behavior into the package. Example threats in this category include building from an unofficial fork or abusing a build parameter to modify the build. Usually expectations identify the canonical source repository (which is the entry in externalParameters) and any other security-relevant external parameters.

The expectations SHOULD cover the following:

Verifiers SHOULD reject unrecognized fields in externalParameters to err on the side of caution. It is acceptable to allow a parameter to have a range of values (possibly any value) if it is known that any value in the range is safe. Implementations need not special-case the buildType if JSON comparisons are sufficient.

Possible models for implementing expectation setting in package ecosystems (not exhaustive):

• Trust on first use: Accept the first version of the package as-is. On each version update, compare the old provenance to the new provenance and alert on any differences. This can be augmented by having rules about what changes are benign, such as a parameter known to be safe or a heuristic about safe git refs.

• Explicit policy: Package producer defines the expectations for the package and distributes it to the verifier; the verifier uses these expectations after verifying their authenticity. In this model, there MUST be some protection against an adversary unilaterally modifying the policy. For example, this might involve two-party control over policy modifications, or having consumers accept each policy change (another form of trust on first use).

• Immutable policy: Expectations for a package cannot change. In this model, the package name is immutably bound to a source repository and all other expectations are defined in the source repository. This is how go works, for example, since the package name is the source repository location.

TIP: Difficulty in setting meaningful expectations for externalParameters can be a sign that the buildType’s level of abstraction is too low. For example, externalParameters that record a list of commands to run is likely impractical to verify because the commands change on every build. Instead, consider a buildType that defines the list of commands in a configuration file in a source repository, then make put only the source repository in externalParameters. Such a design is easier to verify because the source repository is constant across builds.

Step 3: Check dependencies (recursively)

Finally, recursively check the resolvedDependencies as available and to the extent desired. This mitigates threat “E”. While SLSA v1.0 does not have any requirements on the completeness or verification of resolvedDependencies, one might wish to go beyond SLSA’s minimum requirements in order to protect against threats further up the supply chain.

One possible approach is to recursively verify each entry in resolvedDependencies. A Verification Summary Attestation (VSA) can make this process more efficient by recording the result of prior verifications. A trimming heuristic or exception mechanism will almost always be necessary because there will always be some transitive dependencies that are SLSA Build L0. (For example, consider the compiler’s compiler’s compiler’s … compiler.) If resolvedDependencies is incomplete, this can be done on a best-effort basis.

Index of build types

The following is an partial index of build type definitions. Each contains a complete example predicate.

• GitHub Actions Workflow (community-maintained)

TODO: Before marking the spec stable, add at least 1-2 other build types to validate that the design is general enough to apply to other builders.

Migrating from 0.2

To migrate from version 0.2 (old), use the following pseudocode. The meaning of each field is unchanged unless otherwise noted.

{
    "buildDefinition": {
        // The `buildType` MUST be updated for v1.0 to describe how to
        // interpret `inputArtifacts`.
        "buildType": /* updated version of */ old.buildType,
        "externalParameters":
            old.invocation.parameters + {
            // It is RECOMMENDED to rename "entryPoint" to something more
            // descriptive.
            "entryPoint": old.invocation.configSource.entryPoint,
            // It is OPTIONAL to rename "source" to something more descriptive,
            // especially if "source" is ambiguous or confusing.
            "source": old.invocation.configSource.uri,
        },
        "systemParameters": old.invocation.environment,
        "resolvedDependencies":
            old.materials + [
            {
                "uri": old.invocation.configSource.uri,
                "digest": old.invocation.configSource.digest,
            }
        ]
    },
    "runDetails": {
        "builder": {
            "id": old.builder.id,
            "version": null,  // not in v0.2
            "builderDependencies": null,  // not in v0.2
        },
        "metadata": {
            "invocationId": old.metadata.buildInvocationId,
            "startedOn": old.metadata.buildStartedOn,
            "finishedOn": old.metadata.buildFinishedOn,
        },
        "byproducts": null,  // not in v0.2
    },
}

The following fields from v0.2 are no longer present in v1.0:

• entryPoint: Use externalParameters[<name>] instead.
• buildConfig: No longer inlined into the provenance. Instead, either:
  • If the configuration is a top-level input, record its digest in externalParameters["config"].
  • Else if there is a known use case for knowing the exact resolved build configuration, record its digest in byproducts. An example use case might be someone who wishes to parse the configuration to look for bad patterns, such as curl | bash.
  • Else omit it.
• metadata.completeness: Now implicit from builder.id.
• metadata.reproducible: Now implicit from builder.id.

Change history

v1.0 RC1

Major refactor to reduce misinterpretation, including a minor change in model.

• Significantly expanded all documentation.
• Altered the model slightly to better align with real-world build systems, align with reproducible builds, and make verification easier.
• Grouped fields into buildDefinition vs runDetails.
• Renamed, with slight changes in semantics:
  • parameters -> externalParameters
  • environment -> systemParameters
  • materials -> resolvedDependencies
• Removed:
  • configSource: No longer special-cased. Now represented as externalParameters + resolvedDependencies.
  • buildConfig: No longer inlined into the provenance. Can be replaced with a reference in externalParameters or byproducts, depending on the semantics, or omitted if not needed.
  • completeness and reproducible: Now implied by builder.id.
• Added:
  • localName, downloadLocation, and mediaType
  • builder.version
  • byproducts

v0.2

Refactored to aid clarity and added buildConfig. The model is unchanged.

• Replaced definedInMaterial and entryPoint with configSource.
• Renamed recipe to invocation.
• Moved invocation.type to top-level buildType.
• Renamed arguments to parameters.
• Added buildConfig, which can be used as an alternative to configSource to validate the configuration.

rename: slsa.dev/provenance

Renamed to “slsa.dev/provenance”.

v0.1.1

• Added metadata.buildInvocationId.

v0.1

Initial version, named “in-toto.io/Provenance”