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Specification for in-toto attestation layers

Version: v1.1

Index:

  • 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 a subject artifact, with a type-specific schema.
  • Bundle: Defines a method of grouping multiple attestations together.

Common field types

As part of this specification, we also define a set of common field types for any layer of an in-toto attestation.

Parsing rules

The following rules apply to Statement and any Predicate that opt into this model.

  • Unrecognized fields: Consumers MUST ignore unrecognized fields unless otherwise noted in the predicate specification. This is to allow minor version upgrades and extension fields. Ignoring fields is safe due to the monotonic principle.

  • Versioning: Each type has a SemVer2 version number and the TypeURI reflects the major version number. A message is always semantically correct, but possibly incomplete, when parsed as any other version with the same major version number and thus the same TypeURI. Minor version changes always follow the monotonic principle. NOTE: 0.X versions are considered major versions.

  • Extension fields: Producers MAY add extension fields to any JSON object. Extension fields SHOULD use names that are unlikely to collide with names used by other orgs. Producers MAY file an issue/PR to document extension fields they're using. Any consumer MAY parse and use these extensions if desired.

    Field names SHOULD avoid special characters like . and $ as these can make querying these fields in some databases more difficult.

    The presence or absence of the extension field MUST NOT influence the meaning of any other field, and the field MUST follow the monotonic principle.

  • Monotonic principle: A policy is considered monotonic if ignoring an attestation, or a field within an attestation, will never turn a DENY decision into an ALLOW. A predicate or field follows the monotonic principle if the expected policy that consumes it is monotonic. Consumers SHOULD design policies to be monotonic. Example: instead of "deny if a 'has vulnerabilities' attestation exists", prefer "deny unless a 'no vulnerabilities' attestation exists".

See versioning rules for details and examples.