provenance post-processor

The provenance post-processor creates in-toto attestations for an input artifact. It can create Supply-chain Levels for Software Artifacts (SLSA) provenance and software bill of materials (SBOM) attestations. It can also sign them with a PEM key, a key management service, or Sigstore keyless signing.

Example

post-processor "provenance" {
  build_type  = "https://packer.io/buildtypes/hcl2/v1"
  template    = "ubuntu.pkr.hcl"
  only_builds = ["qemu.ubuntu"]
  sbom        = true
}

Configuration

• provenance (boolean) - Whether to emit provenance attestations. Enabled by default; set to false to skip the post-processor and pass the artifact through unchanged.

• build_type (string) - The SLSA buildType URI recorded in the provenance predicate. Defaults to https://packer.io/buildtypes/hcl2/v1.

• output_dir (string) - Directory where attestation sidecar files are written. Defaults to the directory containing the artifact's first file, or the current directory for artifacts without local files.

• template (string) - Path to the Packer template that produced the artifact. Recorded as an external parameter in the provenance predicate.

• only_builds ([]string) - The list of builds this artifact came from. Recorded as an external parameter in the provenance predicate.

• user_variables (map[string]string) - Additional user variables to record as external parameters in the provenance predicate. Values for variables named in packer_sensitive_variables are redacted.

• source_uri (string) - Overrides the auto-detected source repository URI recorded as a resolved dependency. By default the source is detected from the Git repository containing the current working directory or from CI environment variables.

• sbom (bool) - Whether to also generate a software bill of materials (SBOM) and a corresponding SBOM attestation alongside the provenance statement.

• sbom_format (string) - The SBOM output format, either cyclonedx (default) or spdx.

• sbom_scan_path (string) - The path to scan when generating the SBOM. Defaults to the artifact's files or their common parent directory. Required when the artifact files span multiple directories.

• sbom_scope (string) - The SBOM scan scope, either squashed (default) or all-layers.

• sbom_exclude ([]string) - Glob patterns of paths to exclude from the SBOM scan.

• signing_mode (string) - The signing mode for attestations: none (default, unsigned JSON), key (local PEM key), kms (KMS or Vault URI), or keyless (Sigstore Fulcio).

• signer (string) - The signer reference. A PEM private key path for key mode, or a KMS or Vault URI such as awskms://..., gcpkms://..., azurekms://..., or hashivault://... for kms mode.

• key (string) - An alias for signer. When both are set they must be equal.

• verifier (string) - The PEM verifier path used to verify the signature in key and kms modes. Defaults to the signer's derived public key.

• fulcio_url (string) - The Fulcio certificate authority URL used for keyless signing. Defaults to https://fulcio.sigstore.dev.

• rekor_url (string) - The Rekor transparency log URL used when upload_tlog is enabled. Defaults to https://rekor.sigstore.dev.

• upload_tlog (bool) - Whether to upload the keyless signature to the Rekor transparency log and emit a Sigstore bundle carrying the transparency evidence.

• trusted_root_path (string) - An optional path to a Sigstore trusted-root JSON file used to pin keyless verification. When unset, the public Sigstore trusted root is fetched.

• keyless_identity (string) - The expected signing identity for keyless mode, such as the workflow ref https://github.com/OWNER/REPO/.github/workflows/build.yml@refs/heads/main. Required for keyless signing.

• keyless_oidc_issuer (string) - The expected OIDC issuer for keyless mode, such as https://token.actions.githubusercontent.com. Required for keyless signing.

Output files

The post-processor writes sidecar files next to the artifact, or in directory specified in the output_dir argument. The sidecar files are named after the artifact. For example, Packer could creates the following files for an artifact named image:

• image.provenance.json - the SLSA provenance attestation.
• image.provenance.sigstore.json - the Sigstore bundle, written when signing_mode = "keyless".
• image.sbom.cdx.json or image.sbom.spdx.json - the raw SBOM, written when sbom = true.
• image.sbom.att.json - the SBOM attestation, written when sbom = true.

Verify an attestation

Use the packer verify-attestation command to verify a signed attestation. For keyless signing, provide the Sigstore bundle, the expected signing identity and OIDC issuer, and require Rekor verification when your policy requires transparency-log evidence. The following example uses the packer verify-attestation command to verify an attestation using keyless signing:

packer verify-attestation \
  -signing-mode=keyless \
  -bundle=image.provenance.sigstore.json \
  -require-rekor \
  -keyless-identity="https://github.com/OWNER/REPO/.github/workflows/build.yml@refs/heads/main" \
  -keyless-oidc-issuer="https://token.actions.githubusercontent.com" \
  image.provenance.json