Requirements

This document covers all of the detailed requirements for an artifact to meet SLSA. For a broader overview, including basic terminology and threat model, see the overview.

Reminder: SLSA is in alpha. The definitions below are not yet finalized and subject to change, particularly SLSA 3-4.

What is SLSA?

SLSA is a set of incrementally adoptable security guidelines, established by industry consensus. The standards set by SLSA are guiding principles for both software producers and consumers: producers can follow the guidelines to make their software more secure, and consumers can make decisions based on a software package’s security posture. SLSA’s four levels are designed to be incremental and actionable, and to protect against specific integrity attacks. SLSA 4 represents the ideal end state, and the lower levels represent milestones with corresponding integrity guarantees.

Terminology

SLSA’s framework addresses every step of the software supply chain - the sequence of steps resulting in the creation of an artifact. We represent a supply chain as a directed acyclic graph of sources, builds, dependencies, and packages. One artifact’s supply chain is a combination of its dependencies’ supply chains plus its own sources and builds.

Software Supply Chain Model

Term Description Example
Artifact An immutable blob of data; primarily refers to software, but SLSA can be used for any artifact. A file, a git commit, a directory of files (serialized in some way), a container image, a firmware image.
Source Artifact that was directly authored or reviewed by persons, without modification. It is the beginning of the supply chain; we do not trace the provenance back any further. Git commit (source) hosted on GitHub (platform).
Build Process that transforms a set of input artifacts into a set of output artifacts. The inputs may be sources, dependencies, or ephemeral build outputs. .travis.yml (process) run by Travis CI (platform).
Package Artifact that is “published” for use by others. In the model, it is always the output of a build process, though that build process can be a no-op. Docker image (package) distributed on DockerHub (platform). A ZIP file containing source code is a package, not a source, because it is built from some other source, such as a git commit.
Dependency Artifact that is an input to a build process but that is not a source. In the model, it is always a package. Alpine package (package) distributed on Alpine Linux (platform).

Definitions

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.

Immutable reference: An identifier that is guaranteed to always point to the same, immutable artifact. This MUST allow the consumer to locate the artifact and SHOULD include a cryptographic hash of the artifact’s contents to ensure integrity. Examples: git URL + branch/tag/ref + commit ID; cloud storage bucket ID + SHA-256 hash; Subversion URL (no hash).

Platform: Infrastructure or service that hosts the source, build, or distribution of software. Examples: GitHub, Google Cloud Build, Travis CI, Mozilla’s self-hosted Mercurial server.

Provenance: Metadata about how an artifact was produced.

Revision: An immutable, coherent state of a source. In Git, for example, a revision is a commit in the history reachable from a specific branch in a specific repository. Different revisions within one repo MAY have different levels. Example: the most recent revision on a branch meets SLSA 4 but very old historical revisions before the cutoff do not.

Strong authentication: Authentication that maps back to a specific person using an authentication mechanism which is resistant to account and credential compromise. For example, 2-factor authentication (2FA) where one factor is a hardware security key (i.e. YubiKey).

Trusted persons: Set of persons who are granted the authority to maintain a software project. For example, https://github.com/MarkLodato/dotfiles has just one trusted person (MarkLodato), while https://hg.mozilla.org/mozilla-central has a set of trusted persons with write access to the mozilla-central repository.

Source requirements

RequirementDescriptionL1L2L3L4
Version controlled

Every change to the source is tracked in a version control system that meets the following requirements:

  • [Change history] There exists a record of the history of changes that went into the revision. Each change must contain: the identities of the uploader and reviewers (if any), timestamps of the reviews (if any) and submission, the change description/justification, the content of the change, and the parent revisions.

  • [Immutable reference] There exists a way to indefinitely reference this particular, immutable revision. In git, this is the {repo URL + branch/tag/ref + commit ID}.

Most popular version control system meet this requirement, such as git, Mercurial, Subversion, or Perforce.

NOTE: This does NOT require that the code, uploader/reviewer identities, or change history be made public. Rather, some organization must attest to the fact that these requirements are met, and it is up to the consumer whether this attestation is sufficient.

“○” = RECOMMENDED.

○✓✓✓
Verified history

Every change in the revision’s history has at least one strongly authenticated actor identity (author, uploader, reviewer, etc.) and timestamp. It must be clear which identities were verified, and those identities must use two-step verification or similar. (Exceptions noted below.)

  • [First-parent history] In the case of a non-linear version control system, where a revision can have more than one parent, only the “first parent history” is in scope. In other words, when a feature branch is merged back into the main branch, only the merge itself is in scope.
  • [Historical cutoff] There is some TBD exception to allow existing projects to meet SLSA 3/4 even if historical revisions were present in the history. Current thinking is that this could be either last N months or a platform attestation guaranteeing that future changes in the next N months will meet the requirements.
 ✓✓
Retained indefinitely

The revision and its change history are preserved indefinitely and cannot be deleted, except when subject to an established and transparent policy for obliteration, such as a legal or policy requirement.

  • [Immutable history] It must not be possible for persons to delete or modify the history, even with multi-party approval, except by trusted platform admins with two-party approval following the obliterate policy.
  • [Limited retention for SLSA 3] At SLSA 3 (but not 4), it is acceptable for the retention to be limited to 18 months, as attested by the source control platform.
    • Example: If a commit is made on 2020-04-05 and then a retention attestation is generated on 2021-01-01, the commit must be retained until at least 2022-07-01.
 18 mo.✓
Two-person reviewed

Every change in the revision’s history was agreed to by two trusted persons prior to submission, and both of these trusted persons were strongly authenticated. (Exceptions from Verified History apply here as well.)

  • The following combinations are acceptable:
    • Uploader and reviewer are two different trusted persons.
    • Two different reviewers are trusted persons.
  • [Different persons] The platform ensures that no person can use alternate identities to bypass the two-person review requirement.
    • Example: if a person uploads with identity X then reviews with alias Y, the platform understands that this is the same person and does not consider the review requirement satisfied.
  • [Informed review] The reviewer is able and encouraged to make an informed decision about what they’re approving. The reviewer should be presented with a full, meaningful content diff between the proposed revision and the previously reviewed revision. For example, it is not sufficient to just indicate that file changed without showing the contents.
  • [Context-specific approvals] Approvals are for a specific context, such as a repo + branch in git. Moving fully reviewed content from one context to another still requires review. (Exact definition of “context” depends on the project, and this does not preclude well-understood automatic or reviewless merges, such as cutting a release branch.)
    • Git example: If a fully reviewed commit in one repo is merged into a different repo, or a commit in one branch is merged into a different branch, then the merge still requires review.
 ✓

Build requirements

Requirements on build process:

RequirementDescriptionL1L2L3L4
Scripted build

All build steps were fully defined in some sort of “build script”. The only manual command, if any, was to invoke the build script.

Examples:

  • Build script is Makefile, invoked via make all.
  • Build script is .github/workflows/build.yaml, invoked by GitHub Actions.
✓✓✓✓
Build service

All build steps ran using some build service, not on a developer’s workstation.

Examples: GitHub Actions, Google Cloud Build, Travis CI.

 ✓✓✓
Build as code

The build definition and configuration is defined in source control and is executed by the build service.

Examples: cloudbuild.yaml, .github/workflows/build.yaml, zuul.yaml.

 ✓✓
Ephemeral environment

The build service ensured that the build steps ran in an ephemeral environment, such as a container or VM, provisioned solely for this build, and not reused from a prior build.

 ✓✓
Isolated

The build service ensured that the build steps ran in an isolated environment free of influence from other build instances, whether prior or concurrent.

  • It MUST NOT be possible for a build to access any secrets of the build service, such as the provenance signing key.
  • It MUST NOT be possible for two builds that overlap in time to influence one another.
  • It MUST NOT be possible for one build to persist or influence thebuild environment of a subsequent build.
  • Build caches, if used, MUST be purely content-addressable to prevent tampering.
 ✓✓
Parameterless

The build output cannot be affected by user parameters other than the build entry point and the top-level source location. In other words, the build is fully defined through the build script and nothing else.

Examples:

 ✓
Hermetic

All transitive build steps, sources, and dependencies were fully declared up front with immutable references, and the build steps ran with no network access.

The user-defined build script:

  • MUST declare all dependencies, including sources and other build steps, using immutable references in a format that the build service understands.

The build service:

  • MUST fetch all artifacts in a trusted control plane.
  • MUST NOT allow mutable references.
  • MUST verify the integrity of each artifact.
    • If the immutable reference includes a cryptographic hash, the service MUST verify the hash and reject the fetch if the verification fails.
    • Otherwise, the service MUST fetch the artifact over a channel that ensures transport integrity, such as TLS or code signing.
  • MUST prevent network access while running the build steps.
    • This requirement is “best effort.” It SHOULD deter a reasonable team from having a non-hermetic build, but it need not stop a determined adversary. For example, using a container to prevent network access is sufficient.
 ✓
Reproducible

Re-running the build steps with identical input artifacts results in bit-for-bit identical output. Builds that cannot meet this MUST provide a justification why the build cannot be made reproducible.

“○” means that this requirement is “best effort”. The user-provided build script SHOULD declare whether the build is intended to be reproducible or a justification why not. The build service MAY blindly propagate this intent without verifying reproducibility. A consumer MAY reject the build if it does not reproduce.

 ○

Provenance requirements

Requirements on the process by which provenance is generated and consumed:

RequirementDescriptionL1L2L3L4
Available

The provenance is available to the consumer in a format that the consumer accepts. The format SHOULD be in-toto SLSA Provenance, but another format MAY be used if both producer and consumer agree and it meets all the other requirements.

✓✓✓✓
Authenticated

The provenance’s authenticity and integrity can be verified by the consumer. This SHOULD be through a digital signature from a private key accessible only to the service generating the provenance.

 ✓✓✓
Service generated

The data in the provenance MUST be obtained from the build service (either because the generator is the build service or because the provenance generator reads the data directly from the build service).

Regular users of the service MUST NOT be able to inject or alter the contents, except as noted below.

The following provenance fields MAY be generated by the user-controlled build steps:

  • The output artifact hash from Identifies Artifact.
    • Reasoning: This only allows a “bad” build to falsely claim that it produced a “good” artifact. This is not a security problem because the consumer MUST accept only “good” builds and reject “bad” builds.
  • The “reproducible” boolean and justification from Reproducible.
 ✓✓✓
Non-falsifiable

Provenance cannot be falsified by the build service’s users.

NOTE: This requirement is a stricter version of Service Generated.

  • The provenance signing key MUST be stored in a secure key management system accessible only to the build service account.
  • The provenance signing key MUST NOT be accessible to the environment running the user-defined build steps.
  • Every field in the provenance MUST be generated or verified by the build service in a trusted control plane. The user-controlled build steps MUST NOT be able to inject or alter the contents, except as noted below.

The following provenance fields MAY be generated by the user-controlled build steps without the build service verifying their correctness:

  • The output artifact hash from Identifies Artifact.
    • Reasoning: This only allows a “bad” build to falsely claim that it produced a “good” artifact. This is not a security problem because the consumer MUST accept only “good” builds and reject “bad” builds.
  • The “reproducible” boolean and justification from Reproducible.
 ✓✓
Dependencies complete

Provenance records all build dependencies that were available while running the build steps. This includes the initial state of the machine, VM, or container of the build worker.

  • MUST include all user-specified build steps, sources, dependencies.
  • SHOULD include all service-provided artifacts.
 ✓

Requirements on the contents of the provenance:

RequirementDescriptionL1L2L3L4
Identifies artifact

The provenance MUST identify the output artifact via at least one cryptographic hash. The provenance MAY provide multiple identifying cryptographic hashes using different algorithms. When only one hash is provided, the RECOMMENDED algorithm is SHA-256 for cross-system compatibility. If another algorithm is used, it SHOULD be resistant to collisions and second preimages.

✓✓✓✓
Identifies builder

The provenance identifies the entity that performed the build and generated the provenance. This represents the entity that the consumer must trust. Examples: “GitHub Actions with a GitHub-hosted worker”, “jdoe@example.com’s machine”.

✓✓✓✓
Identifies build instructions

The provenance identifies the top-level instructions used to execute the build.

The identified instructions SHOULD be at the highest level available to the build (e.g. if the build is told to run build.sh it should list build.sh and NOT the individual instructions in build.sh).

If build-as-code is used, this SHOULD be the source repo and entry point of the build config (as in the GitHub Actions example).

If the build isn’t defined in code it MAY list the details of what it was asked to do (as in the Google Cloud Build RPC example or the Explicitly Run Commands example).

✓✓✓✓
Identifies source code

The provenance identifies the repository origin(s) for the source code used in the build.

The identified repositories SHOULD only include source used directly in the build. The source of dependencies SHOULD NOT be included.

At level 2 this information MAY come from users and DOES NOT need to be authenticated by the builder.

At level 3+ this information MUST be authenticated by the builder (i.e. the builder either needs to have fetched the source itself or observed the fetch).

At level 4 this information MUST be complete (i.e. all source repositories used in the build are listed).

✓✓ (Authenticated)✓ (Complete)
Identifies entry point

The provenance identifies the “entry point” of the build definition (see build-as-code) used to drive the build including what source repo the configuration was read from.

Example:

  • source repo: git URL + branch/tag/ref + commit ID
  • entrypoint: path to config file(s) (e.g. ./.zuul.yaml) + job name within config (e.g. envoy-build-arm64)
✓✓
Includes all build parameters

The provenance includes all build parameters under a user’s control. See Parameterless for details. (At L3, the parameters must be listed; at L4, they must be empty.)

 ✓✓
Includes all transitive dependencies

The provenance includes all transitive dependencies listed in Dependencies Complete.

 ✓
Includes reproducible info

The provenance includes a boolean indicating whether build is intended to be reproducible and, if so, all information necessary to reproduce the build. See Reproducible for more details.

 ✓
Includes metadata

The provenance includes metadata to aid debugging and investigations. This SHOULD at least include start and end timestamps and a permalink to debug logs.

“○” = RECOMMENDED.

○○○○

Common requirements

Common requirements for every trusted system involved in the supply chain (source, build, distribution, etc.)

RequirementDescriptionL1L2L3L4
Security

The system meets some TBD baseline security standard to prevent compromise. (Patching, vulnerability scanning, user isolation, transport security, secure boot, machine identity, etc. Perhaps NIST 800-53 or a subset thereof.)

 ✓
Access

All physical and remote access must be rare, logged, and gated behind multi-party approval.

 ✓
Superusers

Only a small number of platform admins may override the guarantees listed here. Doing so MUST require approval of a second platform admin.

 ✓

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