Producing artifacts

This page covers the detailed technical requirements for producing artifacts at each SLSA level. The intended audience is system implementers and security engineers.

For an informative description of the levels intended for all audiences, see Levels. For background, see Terminology. To better understand the reasoning behind the requirements, see Threats and mitigations.

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.

Overview

Build levels

In order to produce artifacts with a specific build level, responsibility is split between the Producer and Build system. The build system MUST strengthen the security controls in order to achieve a specific level while the producer MUST choose and adopt a build system capable of achieving a desired build level, implementing any controls as specified by the chosen system.

Implementer	Requirement	Degree	L1	L2	L3
Producer	Choose an appropriate build system		✓	✓	✓
	Follow a consistent build process		✓	✓	✓
	Distribute provenance		✓	✓	✓
Build system	Provenance generation	Exists	✓	✓	✓
		Authentic		✓	✓
		Unforgeable			✓
	Isolation strength	Build service		✓	✓
	Isolation strength	Isolated			✓

Security Best Practices

While the exact definition of what constitutes a secure system is beyond the scope of this specification, all implementations MUST use industry security best practices to be conformant to this specification. This includes, but is not limited to, using proper access controls, securing communications, implementing proper management of cryptographic secrets, doing frequent updates, and promptly fixing known vulnerabilities.

Various relevant standards and guides can be consulted for that matter such as the CIS Critical Security Controls.

Producer

A package’s producer is the organization that owns and releases the software. It might be an open-source project, a company, a team within a company, or even an individual.

NOTE: There were more requirements for producers in the initial draft version (v0.1) which impacted how a package can be built. These were removed in the v1.0 specification and will be reassessed and re-added as indicated in the future directions.

Choose an appropriate build system

The producer MUST select a build system that is capable of reaching their desired SLSA Build Level.

For example, if a producer wishes to produce a Build Level 3 artifact, they MUST choose a builder capable of producing Build Level 3 provenance.

Follow a consistent build process

The producer MUST build their artifact in a consistent manner such that verifiers can form expectations about the build process. In some implementations, the producer MAY provide explicit metadata to a verifier about their build process. In others, the verifier will form their expectations implicitly (e.g. trust on first use).

If a producer wishes to distribute their artifact through a package ecosystem that requires explicit metadata about the build process in the form of a configuration file, the producer MUST complete the configuration file and keep it up to date. This metadata might include information related to the artifact’s source repository and build parameters.

Distribute provenance

The producer MUST distribute provenance to artifact consumers. The producer MAY delegate this responsibility to the package ecosystem, provided that the package ecosystem is capable of distributing provenance.

Build system

A package’s build system is the infrastructure used to transform the software from source to package. This includes the transitive closure of all hardware, software, persons, and organizations that can influence the build. A build system is often a hosted, multi-tenant build service, but it could be a system of multiple independent rebuilders, a special-purpose build system used by a single software project, or even an individual’s workstation. Ideally, one build system is used by many different software packages so that consumers can minimize the number of trusted systems. For more background, see Build Model.

The build system is responsible for providing two things: provenance generation and isolation between builds. The Build level describes the degree to which each of these properties is met.

Provenance generation

The build system is responsible for generating provenance describing how the package was produced.

The SLSA Build level describes the overall provenance integrity according to minimum requirements on its:

Completeness: What information is contained in the provenance?
Authenticity: How strongly can the provenance be tied back to the builder?
Accuracy: How resistant is the provenance generation to tampering within the build process?

Requirement Description L1 L2 L3

Provenance Exists

Requirement	Description	L1	L2	L3
Provenance Exists	The build process MUST generate provenance that unambiguously identifies the output package by cryptographic digest and describes how that package was produced. The format MUST be acceptable to the package ecosystem and/or consumer. It is RECOMMENDED to use the SLSA Provenance format and associated suite because it is designed to be interoperable, universal, and unambiguous when used for SLSA. See that format’s documentation for requirements and implementation guidelines. If using an alternate format, it MUST contain the equivalent information as SLSA Provenance at each level and SHOULD be bi-directionally translatable to SLSA Provenance. Completeness: Best effort. The provenance at L1 SHOULD contain sufficient information to catch mistakes and simulate the user experience at higher levels in the absence of tampering. In other words, the contents of the provenance SHOULD be the same at all Build levels, but a few fields MAY be absent at L1 if they are prohibitively expensive to implement. Authenticity: No requirements. Accuracy: No requirements.	✓	✓	✓
Provenance is Authentic	Authenticity: Consumers MUST be able to validate the authenticity of the provenance attestation in order to: Ensure integrity: Verify that the digital signature of the provenance attestation is valid and the provenance was not tampered with after the build. Define trust: Identify the build system and other entities that are necessary to trust in order to trust the artifact they produced. This SHOULD be through a digital signature from a private key accessible only to the service that generated the provenance attestation. This allows the consumer to trust the contents of the provenance attestation, such as the identity of the build system. Accuracy: The provenance MUST be generated by the build system (i.e. within the trust boundary identified in the provenance) and not by a tenant of the build system (i.e. outside the trust boundary), except as noted below. 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. The build system MUST have some security control to prevent tenants from tampering with the provenance. However, there is no minimum bound on the strength. The purpose is to deter adversaries who might face legal or financial risk from evading controls. Exceptions for fields that MAY be generated by a tenant of the build system: The names and cryptographic digests of the output artifacts, i.e. `subject` in SLSA Provenance. See forge output digest of the provenance for explanation of why this is acceptable. Any field that is not marked as REQUIRED for Build L2. For example, `resolvedDependencies` in SLSA Provenance MAY be tenant-generated at Build L2. Builders SHOULD document any such cases of tenant-generated fields. Completeness: SHOULD be complete. There MAY be external parameters that are not sufficiently captured in the provenance. Completeness of resolved dependencies is best effort.		✓	✓
Provenance is Unforgeable	Accuracy: Provenance MUST be strongly resistant to forgery by tenants. Any secret material used for authenticating the provenance, for example the signing key used to generate a digital signature, MUST be stored in a secure management system appropriate for such material and accessible only to the build service account. Such secret material 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 in Provenance is Authentic. (Build L3 does not require additional fields beyond those of L2.) Completeness: SHOULD be complete. External parameters MUST be fully enumerated. Completeness of resolved dependencies is best effort. Note: This requirement was called “non-falsifiable” in the initial draft version (v0.1).			✓

The build process MUST generate provenance that unambiguously identifies the output package by cryptographic digest and describes how that package was produced. The format MUST be acceptable to the package ecosystem and/or consumer.

It is RECOMMENDED to use the SLSA Provenance format and associated suite because it is designed to be interoperable, universal, and unambiguous when used for SLSA. See that format’s documentation for requirements and implementation guidelines.

If using an alternate format, it MUST contain the equivalent information as SLSA Provenance at each level and SHOULD be bi-directionally translatable to SLSA Provenance.

Completeness: Best effort. The provenance at L1 SHOULD contain sufficient information to catch mistakes and simulate the user experience at higher levels in the absence of tampering. In other words, the contents of the provenance SHOULD be the same at all Build levels, but a few fields MAY be absent at L1 if they are prohibitively expensive to implement.
Authenticity: No requirements.
Accuracy: No requirements.

✓

Provenance is Authentic

Authenticity: Consumers MUST be able to validate the authenticity of the provenance attestation in order to:

Ensure integrity: Verify that the digital signature of the provenance attestation is valid and the provenance was not tampered with after the build.
Define trust: Identify the build system and other entities that are necessary to trust in order to trust the artifact they produced.

This SHOULD be through a digital signature from a private key accessible only to the service that generated the provenance attestation.

This allows the consumer to trust the contents of the provenance attestation, such as the identity of the build system.

Accuracy: The provenance MUST be generated by the build system (i.e. within the trust boundary identified in the provenance) and not by a tenant of the build system (i.e. outside the trust boundary), except as noted below.

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.
The build system MUST have some security control to prevent tenants from tampering with the provenance. However, there is no minimum bound on the strength. The purpose is to deter adversaries who might face legal or financial risk from evading controls.
Exceptions for fields that MAY be generated by a tenant of the build system:
- The names and cryptographic digests of the output artifacts, i.e. subject in SLSA Provenance. See forge output digest of the provenance for explanation of why this is acceptable.
- Any field that is not marked as REQUIRED for Build L2. For example, resolvedDependencies in SLSA Provenance MAY be tenant-generated at Build L2. Builders SHOULD document any such cases of tenant-generated fields.

Completeness: SHOULD be complete.

There MAY be external parameters that are not sufficiently captured in the provenance.
Completeness of resolved dependencies is best effort.

✓

Provenance is Unforgeable

Accuracy: Provenance MUST be strongly resistant to forgery by tenants.

Any secret material used for authenticating the provenance, for example the signing key used to generate a digital signature, MUST be stored in a secure management system appropriate for such material and accessible only to the build service account.
Such secret material 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 in Provenance is Authentic. (Build L3 does not require additional fields beyond those of L2.)

Completeness: SHOULD be complete.

External parameters MUST be fully enumerated.
Completeness of resolved dependencies is best effort.

Note: This requirement was called “non-falsifiable” in the initial draft version (v0.1).

✓

Isolation strength

The build system is responsible for isolating between builds, even within the same tenant project. In other words, how strong of a guarantee do we have that the build really executed correctly, without external influence?

The SLSA Build level describes the minimum bar for isolation strength. For more information on assessing a build system’s isolation strength, see Verifying build systems.

Requirement Description L1 L2 L3

Build service

Requirement	Description	L1	L2	L3
Build service	All build steps ran using some build service, not on an individual’s workstation. Examples: GitHub Actions, Google Cloud Build, Travis CI.		✓	✓
Isolated	The build service ensured that the build steps ran in an isolated environment, free of unintended external influence. In other words, any external influence on the build was specifically requested by the build itself. This MUST hold true even between builds within the same tenant project. The build system MUST guarantee the following: It MUST NOT be possible for a build to access any secrets of the build service, such as the provenance signing key, because doing so would compromise the authenticity of the provenance. It MUST NOT be possible for two builds that overlap in time to influence one another, such as by altering the memory of a different build process running on the same machine. It MUST NOT be possible for one build to persist or influence the build environment of a subsequent build. In other words, an ephemeral build environment MUST be provisioned for each build. It MUST NOT be possible for one build to inject false entries into a build cache used by another build, also known as “cache poisoning”. In other words, the output of the build MUST be identical whether or not the cache is used. The build system MUST NOT open services that allow for remote influence unless all such interactions are captured as `externalParameters` in the provenance. There are no sub-requirements on the build itself. Build L3 is limited to ensuring that a well-intentioned build runs securely. It does not require that build systems prevent a producer from performing a risky or insecure build. In particular, the “Isolated” requirement does not prohibit a build from calling out to a remote execution service or a “self-hosted runner” that is outside the trust boundary of the build platform. NOTE: This requirement was split into “Isolated” and “Ephemeral Environment” in the initial draft version (v0.1). NOTE: This requirement is not to be confused with “Hermetic”, which roughly means that the build ran with no network access. Such a requirement requires substantial changes to both the build system and each individual build, and is considered in the future directions.			✓

All build steps ran using some build service, not on an individual’s workstation.

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

✓

Isolated

The build service ensured that the build steps ran in an isolated environment, free of unintended external influence. In other words, any external influence on the build was specifically requested by the build itself. This MUST hold true even between builds within the same tenant project.

The build system MUST guarantee the following:

It MUST NOT be possible for a build to access any secrets of the build service, such as the provenance signing key, because doing so would compromise the authenticity of the provenance.
It MUST NOT be possible for two builds that overlap in time to influence one another, such as by altering the memory of a different build process running on the same machine.
It MUST NOT be possible for one build to persist or influence the build environment of a subsequent build. In other words, an ephemeral build environment MUST be provisioned for each build.
It MUST NOT be possible for one build to inject false entries into a build cache used by another build, also known as “cache poisoning”. In other words, the output of the build MUST be identical whether or not the cache is used.
The build system MUST NOT open services that allow for remote influence unless all such interactions are captured as externalParameters in the provenance.

There are no sub-requirements on the build itself. Build L3 is limited to ensuring that a well-intentioned build runs securely. It does not require that build systems prevent a producer from performing a risky or insecure build. In particular, the “Isolated” requirement does not prohibit a build from calling out to a remote execution service or a “self-hosted runner” that is outside the trust boundary of the build platform.

NOTE: This requirement was split into “Isolated” and “Ephemeral Environment” in the initial draft version (v0.1).

NOTE: This requirement is not to be confused with “Hermetic”, which roughly means that the build ran with no network access. Such a requirement requires substantial changes to both the build system and each individual build, and is considered in the future directions.

✓

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