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"

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 a particular build platform produced a set of software artifacts through execution of the buildDefinition.

The model is as follows:

• Each build runs as an independent process on a multi-tenant build 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 platforms.)

  • The build platform implementer SHOULD define a security model for the build platform in order to clearly identify the platform’s boundaries, actors, and interfaces. This model SHOULD then be used to identify the transitive closure of the trusted build platform for the builder.id as well as the trusted control plane.

• The build process is defined by a parameterized template, identified by buildType. This encapsulates the process that ran, regardless of what platform 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.

  • internalParameters: 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.
• Unset, null, and empty field values MUST be interpreted equivalently.

Schema

Summary

NOTE: This summary (in cue) is informative. In the event of a disagreement with the text description, the text is authoritative.

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

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

#ResourceDescriptor: {
    "uri": string,
    "digest": {
        "sha256": string,
        "sha512": string,
        "gitCommit": string,
        [string]: string,
    },
    "name": string,
    "downloadLocation": string,
    "mediaType": string,
    "content": bytes, // base64-encoded
    "annotations": {
        [string]: _  // any JSON type
    }
}

#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 internal_parameters = 3;
  repeated ResourceDescriptor resolved_dependencies = 4;
}

message ResourceDescriptor {
  string uri = 1;
  map<string, string> digest = 2;
  string name = 3;
  string download_location = 4;
  string media_type = 5;
  bytes content = 6;
  map<string, google.protobuf.Value> annotations = 7;
}

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

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

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

Provenance

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

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 internalParameters 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 internalParameters.

The parameters SHOULD only contain the actual values passed in through the interface to the build platform. 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": {"gitCommit": "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 platform 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 platform 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”.

RunDetails

REQUIRED for SLSA Build L1: builder

Builder

REQUIRED for SLSA Build L1: id

The build platform, or builder for short, represents the transitive closure of all the entities that are, by necessity, trusted to faithfully run the build and record the provenance. This includes not only the software but the hardware and people involved in running the service. For example, a particular instance of Tekton could be a build platform, while Tekton itself is not. For more info, see Build model.

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.

BuildMetadata

REQUIRED: (none)

Extension fields

Implementations MAY add extension fields to any JSON object to describe information that is not captured in a standard field. Guidelines:

• Extension fields SHOULD use names of the form <vendor>_<fieldname>, e.g. examplebuilder_isCodeReviewed. This practice avoids field name collisions by namespacing each vendor. Non-extension field names never contain an underscore.
• Extension fields MUST NOT alter the meaning of any other field. In other words, an attestation with an absent extension field MUST be interpreted identically to an attestation with an unrecognized (and thus ignored) extension field.
• Extension fields SHOULD follow the monotonic principle, meaning that deleting or ignoring the extension SHOULD NOT turn a DENY decision into an ALLOW.

Verification

Please see Verifying Artifacts for a detailed discussion of provenance verification.

Index of build types

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

• GitHub Actions Workflow (community-maintained)
• Google Cloud Build (community-maintained)

To add an entry here, please send a pull request on GitHub.

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,
        },
        "internalParameters": old.invocation.environment,
        "resolvedDependencies":
            old.materials + [
            {
                "uri": old.invocation.configSource.uri,
                "digest": old.invocation.configSource.digest,
            }
        ]
    },
    "runDetails": {
        "builder": {
            "id": old.builder.id,
            "builderDependencies": null,  // not in v0.2
            "version": 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

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 platforms, align with reproducible builds, and make verification easier.
• Grouped fields into buildDefinition vs runDetails.
• Renamed:
  • parameters -> externalParameters (slight change in semantics)
  • environment -> internalParameters (slight change in semantics)
  • materials -> resolvedDependencies (slight change in semantics)
  • buildInvocationId -> invocationId
  • buildStartedOn -> startedOn
  • buildFinishedOn -> finishedOn
• 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:
  • ResourceDescriptor: annotations, content, downloadLocation, mediaType, name
  • Builder: builderDependencies and version
  • byproducts
• Changed naming convention for extension fields.

Differences from RC1 and RC2:

• Renamed systemParameters (RC1 + RC2) -> internalParameters (final).
• Changed naming convention for extension fields (in RC2).
• Renamed localName (RC1) -> name (RC2).
• Added annotations and content (in RC2).

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”