In-toto Attestations

This repository defines the in-toto attestation format, which represents authenticated metadata about a set of software artifacts. Attestations are intended for consumption by automated policy engines, such as in-toto and Binary Authorization.

IMPORTANT

This format is still in development and is not yet used by in-toto (see ITE-6). Versions 0.x are unstable and are subject to change until 1.0. Use git tags to switch between versions.

Latest stable version: 0.1.0

Introduction

An in-toto attestation is authenticated metadata about one or more software artifacts, as per the SLSA Attestation Model.

Examples of hypothetical attestations:

• Provenance: GitHub Actions attests to the fact that it built a container image with digest "sha256:87f7fe…" from git commit "f0c93d…" in the "main" branch of "https://github.com/example/foo".
• Test result: GitHub Actions attests to the fact that the npm tests passed on git commit "f0c93d…".
• Vulnerability scan: Google Container Analysis attests to the fact that no vulnerabilities were found in container image "sha256:87f7fe…" at a particular time.
• Policy decision: Binary Authorization attests to the fact that container image "sha256:87f7fe…" is allowed to run under GKE project "example-project" within the next 4 hours, and that it used the four attestations above and as well as the policy with sha256 hash "79e572" to make its decision.

Goals:

• Standardize artifact metadata without being specific to the producer or consumer. This way CI/CD pipelines, vulnerability scanners, and other systems can generate a single set of attestations that can be consumed by anyone, such as in-toto or Binary Authorization.
• Make it possible to write automated policies that take advantage of structured information.
• Fit within the SLSA Framework.

Tooling / how to use

None yet!

Specification

See spec/README.md. Summary:

• Envelope: Handles authentication and serialization.
• Statement: Binds the attestation to a particular subject and unambiguously identifies the types of the predicate.
• Predicate: Contains arbitrary metadata about the subject, with a type-specific schema.
• Bundle: Defines a method of grouping multiple attestations together.

The processing model provides pseudocode showing how these layers fit together.

Examples

Provenance example

A SLSA Provenance-type attestation describing how the curl 7.72.0 source tarballs were built, pretending they were built on GitHub Actions.

{
  "payloadType": "application/vnd.in-toto+json",
  "payload": "ewogICJzdWJqZWN0IjogWwogICAg...",
  "signatures": [{"sig": "MeQyap6MyFyc9Y..."}]
}

where payload base64-decodes as the following Statement:

{
  "_type": "https://in-toto.io/Statement/v0.1",
  "subject": [
    { "name": "curl-7.72.0.tar.bz2",
      "digest": { "sha256": "ad91970864102a59765e20ce16216efc9d6ad381471f7accceceab7d905703ef" }},
    { "name": "curl-7.72.0.tar.gz",
      "digest": { "sha256": "d4d5899a3868fbb6ae1856c3e55a32ce35913de3956d1973caccd37bd0174fa2" }},
    { "name": "curl-7.72.0.tar.xz",
      "digest": { "sha256": "0ded0808c4d85f2ee0db86980ae610cc9d165e9ca9da466196cc73c346513713" }},
    { "name": "curl-7.72.0.zip",
      "digest": { "sha256": "e363cc5b4e500bfc727106434a2578b38440aa18e105d57576f3d8f2abebf888" }}
  ],
  "predicateType": "https://slsa.dev/provenance/v0.1",
  "predicate": {
    "builder": { "id": "https://github.com/Attestations/GitHubHostedActions@v1" },
    "recipe": {
      "type": "https://github.com/Attestations/GitHubActionsWorkflow@v1",
      "definedInMaterial": 0,
      "entryPoint": "build.yaml:maketgz"
    },
    "metadata": {
      "buildStartedOn": "2020-08-19T08:38:00Z"
    },
    "materials": [
      {
        "uri": "git+https://github.com/curl/curl-docker@master",
        "digest": { "sha1": "d6525c840a62b398424a78d792f457477135d0cf" }
      }, {
        "uri": "github_hosted_vm:ubuntu-18.04:20210123.1"
      }, {
        "uri": "git+https://github.com/actions/checkout@v2",
        "digest": {"sha1": "5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f"}
      }, {
        "uri": "git+https://github.com/actions/upload-artifact@v2",
        "digest": { "sha1": "e448a9b857ee2131e752b06002bf0e093c65e571" }
      }, {
        "uri": "pkg:deb/debian/stunnel4@5.50-3?arch=amd64",
        "digest": { "sha256": "e1731ae217fcbc64d4c00d707dcead45c828c5f762bcf8cc56d87de511e096fa" }
      }, {
        "uri": "pkg:deb/debian/python-impacket@0.9.15-5?arch=all",
        "digest": { "sha256": "71fa2e67376c8bc03429e154628ddd7b196ccf9e79dec7319f9c3a312fd76469" }
      }, {
        "uri": "pkg:deb/debian/libzstd-dev@1.3.8+dfsg-3?arch=amd64",
        "digest": { "sha256": "91442b0ae04afc25ab96426761bbdf04b0e3eb286fdfbddb1e704444cb12a625" }
      }, {
        "uri": "pkg:deb/debian/libbrotli-dev@1.0.7-2+deb10u1?arch=amd64",
        "digest": { "sha256": "05b6e467173c451b6211945de47ac0eda2a3dccb3cc7203e800c633f74de8b4f" }
      }
    ]
  }
}

Custom-type examples

In many cases, custom-type predicates would be a more natural fit, as shown below. Such custom attestations are not yet supported by in-toto because the layout format has no way to reference such attestations. Still, we show the examples to explain the benefits for the new link format.

The initial step is often to write code. This has no materials and no real command. The existing Link schema has little benefit. Instead, a custom predicateType would avoid all of the meaningless boilerplate fields.

(Only the base64-decoded payload Statement is shown, since the outer Envelope looks the same in all cases.)

{
  "_type": "https://in-toto.io/Statement/v0.1",
  "subject": [
    { "digest": { "sha1":  "859b387b985ea0f414e4e8099c9f874acb217b94" } }
  ],
  "predicateType": "https://example.com/CodeReview/v1",
  "predicate": {
    "repo": {
      "type": "git",
      "uri": "https://github.com/example/my-project",
      "branch": "main"
    },
    "author": "mailto:alice@example.com",
    "reviewers": ["mailto:bob@example.com"]
  }
}

Test results are also an awkward fit for the Link schema, since the subject is really the materials, not the products. Again, a custom predicateType is a better fit:

{
  "_type": "https://in-toto.io/Statement/v0.1",
  "subject": [
    { "digest": { "sha1": "859b387b985ea0f414e4e8099c9f874acb217b94" } }
  ],
  "predicateType": "https://example.com/TestResult/v1",
  "predicate": {
    "passed": true
  }
}

Motivation

This project has two main goals:

1. Support use cases where the existing link schema is a poor fit. For example, test steps and vulnerability scans are not about "producing" a new artifact so they are awkward to represent in the current format.
2. Support interoperability with Binary Authorization, which will support the agreed-upon format once finalized. This way we have a single ecosystem of software supply chain security.

Functional requirements:

• Must support user-defined types and schemas, for two reasons:
  • To allow in-toto users to more naturally express attestations, as explained above.
  • Because Binary Authorization does not want to require its users to use the existing in-toto link schema, which is overly specific.
• Should allow indexing of attestations by artifact ID, without having to understand the user-defined schema.
  • Primary reason: To support generic attestation indexing/storage/fetching without requiring user configuration for every type of attestation.
  • Secondary reason: To simplify the programming model of policies. The binding between artifact and attestation can be done in a framework without requiring type-dependent configuration.
  • Implication: the association between attestation and primary artifact ID must be standardized.
• Should allow identification of related artifacts IDs given an attestation, without having to understand the user-defined schema.
  • Reason: To support "inline attestations," where the client fetches and sends all required attestations to the server for verification. The client does not know the policy ahead of time or understand all attestation types.
  • Example: Given a provenance attestation for a docker image, it should be possible to identify all the materials generically.
  • Implication: the association between attestation and related artifact IDs must be standardized.

Nonfunctional requirements:

• Must support backwards compatible links that can be consumed by existing layout files.
• Must differentiate between different types of related artifacts (only if related artifacts are standardized.) Examples: materials vs products, sources vs build tools.
  • Should be type-dependent, rather than mandating "materials" and "products."

Reasoning

Reason for separate Statement and Predicate layers

The Statement layer has a fixed schema while the Predicate layer has an arbitrary schema. Furthermore, the fixed Statement schema has a subject and predicateType. There are two main reasons for this.

First, doing so allows policy engines to make decisions without requiring Predicate-specific logic or configuration. Binary Authorization policies today are purely about "does an attestation exist that is signed by X with subject Y", and similarly in-toto layouts are about "does an attestation exist that is signed by X with materials/products Z?"[1] These relatively simple policies are quite powerful. With this proposal, such policies become more expressive without any additional configuration: "does an attestation exist that is signed by X having predicate type T, with subject Y/Z?"

Second, it enables lookup of attestations by subject, again without Predicate-specific logic or configuration. Consider the policy described in the motivating use case. There, the instruction is "fetch attestations for artifact X". The lookup could be from a set of attestations provided by the caller, or it could be from an external database keyed by subject.[2] Without a standardized subject field, this would be significantly harder.

The alternative is to not have a fixed Statement schema and instead have subject be part of the Predicate. Doing so would require users to configure the system for every possible Predicate type they wanted to support, in order to instruct the system how to find the subject. Furthermore, because there would be no standardization, concepts and models may not necessarily translate between predicate types. For example, one predicate type might require an "or" between artifact IDs, while another requires an "and." This difference would add complexity and confusion.

Footnotes

[1]: The expected_command is only a warning, and inspections require running external commands which is infeasible in many situations.

[2]: That said, we strongly recommend against keying a database purely by content hash. The reason is that such databases quickly run into scaling issues, as explained in Building Secure and Reliable Systems, Chapter 14, page 328, "Ensure Unambiguous Provenance." Instead, we recommend keying primarily by resource name, in addition to content hash.