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Before diving into the SLSA Levels, we need to establish a core set of terminology and models to describe what we’re protecting.

Software supply chain

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).

Build model

We model a build as running on a multi-tenant platform, where each execution is independent. A tenant defines the build, including the input source artifact and the steps to execute. In response to an external trigger, the platform runs the build by initializing the environment, fetching the source and possibly some dependencies, and then starting execution inside the environment. The build then performs arbitrary steps, possibly fetching additional dependencies, and outputs one or more artifacts.

Model Build

Term Description
Platform System that allows tenants to run build. Technically, it is the transitive closure of software and services that must be trusted to faithfully execute the build.
Service A platform that is hosted, not a developer’s machine. (Term used in requirements.)
Build Process that converts input sources and dependencies into output artifacts, defined by the tenant and executed within a single environment.
Steps The set of actions that comprise a build, defined by the tenant.
Environment Machine, container, VM, or similar in which the build runs, initialized by the platform. In the case of a distributed build, this is the collection of all such machines/containers/VMs that run steps.
Trigger External event or request causing the platform to run the build.
Source Top-level input artifact required by the build.
Dependencies Additional input artifacts required by the build.
Outputs Collection of artifacts produced by the build.
Admin Person with administrative access to the platform, potentially allowing them to tamper with the build process or access secret material.
Example: GitHub Actions
Term Example
Platform GitHub Actions + runner + runner’s dependent services
Build Workflow or job (either would be OK)
Steps steps
Environment runs-on
Trigger workflow trigger
Source git commit defining the workflow
Dependencies any other artifacts fetched during execution
Admin GitHub personnel
Example: Distributed Bazel Builds

Suppose a Bazel build runs on GitHub Actions using Bazel’s remote execution feature. Some steps (namely bazel itself) run on a GitHub Actions runner while other steps (Bazel actions) run on a remote execution service.

In this case, the build’s environment is the union of the GitHub Actions runner environment plus the remote execution environment.

Example: Local Builds

The model can still work for the case of a developer building on their local workstation, though this does not meet SLSA 2+.

Term Example
Platform developer’s workstation
Build whatever they ran
Steps whatever they ran
Environment developer’s workstation
Trigger commands that the developer ran
Admin developer