Provenance Post-Processor

The provenance post-processor writes in-toto attestations for the incoming artifact.

It supports:

• SLSA provenance statements enriched with source-control and CI metadata.
• SBOM sidecars plus SBOM attestations.
• Optional DSSE signing with a configurable signer and verifier.

Signing defaults to none, which writes the unsigned JSON statement.

Source detection

The provenance statement records the build's source repository as a resolved dependency. Packer detects this from the Git repository containing the current working directory (the directory Packer runs in). Set source_uri to override the detected value, for example when the build runs outside the source checkout or the remote URL should be normalized.

Signing modes:

• key signs with a PEM private key and verifies with either the signer's derived public key or an explicit verifier PEM.
• kms signs with a KMS or Vault URI such as awskms://..., gcpkms://..., azurekms://..., or hashivault://.... Verification uses the fetched KMS public key or an explicit verifier PEM.
• keyless signs with an ephemeral keypair and a Fulcio-issued certificate. It requires an ambient OIDC token, such as SIGSTORE_ID_TOKEN, or a CI provider token that can be exchanged for a Sigstore identity. When using the built-in verifier path, also configure the expected signing identity and OIDC issuer. Set an optional trusted-root JSON path to pin verification to a specific Sigstore root; otherwise the public Sigstore trusted root is fetched. Keyless signing can also emit a Sigstore bundle sidecar and upload to Rekor.

Example:

post-processor "provenance" {
  build_type = "https://packer.io/buildtypes/hcl2/v1"
  template   = "ubuntu.pkr.hcl"
  only_builds = ["qemu.ubuntu"]
  user_variables = {
    region = "us-east-1"
  }
  sbom = true
}

Signed example:

post-processor "provenance" {
  signing_mode = "key"
  signer       = "keys/provenance-signing.pem"
  verifier     = "keys/provenance-signing.pub.pem"
  sbom         = true
}

KMS example:

post-processor "provenance" {
  signing_mode = "kms"
  signer       = "awskms://alias/packer-provenance"
  sbom         = true
}

Keyless example:

post-processor "provenance" {
  signing_mode        = "keyless"
  fulcio_url          = "https://fulcio.sigstore.dev"
  rekor_url           = "https://rekor.sigstore.dev"
  upload_tlog         = true
  keyless_identity    = "https://github.com/hashicorp/packer/.github/workflows/build.yml@refs/heads/main"
  keyless_oidc_issuer = "https://token.actions.githubusercontent.com"
  trusted_root_path   = "sigstore-trusted-root.json"
}

With keyless signing, Packer writes an additional *.sigstore.json sidecar next to each signed attestation. When upload_tlog = true, that bundle includes Rekor-backed transparency evidence for packer verify-attestation -bundle ....

During a build, keyless verification enforces the Fulcio certificate chain and the configured identity policy. For Rekor-backed verification, run packer verify-attestation with the generated Sigstore bundle and the -require-rekor and/or -require-timestamp flags.

SLSA levels and CI

SLSA Build levels are mostly properties of the build platform, not the build tool. Packer is a tool, so its reach is:

Packer generates SLSA Provenance v1 and reaches Build L1, and L2 when run on a hosted CI with keyless signing. L3 is a property of the build platform: it requires the signing key to be unreachable by the build steps. Packer does not confer L3 on its own, but the delegated-signing pattern is compatible with an L3 platform.

Reference GitHub Actions workflows for both the L2 keyless pattern and the L3-compatible delegated-signing pattern live under examples/ci/.