jmgilman · Jun 28, 2026 · Jun 28, 2026
diff --git a/.github/scripts/stage_ghd_release_assets.py b/.github/scripts/stage_ghd_release_assets.py
@@ -127,7 +127,7 @@ def validate_ghd_config(config: dict[str, Any], *, binary_name: str, repository:
     if not isinstance(provenance, dict):
         raise StageError("ghd.toml must define a [provenance] table")
 
-    expected_signer = f"{repository}/.github/workflows/release.yml"
+    expected_signer = f"{repository}/.github/workflows/attest.yml"
     actual_signer = provenance.get("signer_workflow")
     if actual_signer != expected_signer:
         raise StageError(

diff --git a/.github/scripts/test_stage_ghd_release_assets.py b/.github/scripts/test_stage_ghd_release_assets.py
@@ -126,7 +126,7 @@ def run_script(root: Path) -> tuple[int, str, str]:
 @contextlib.contextmanager
 def fixture(
     *,
-    signer: str = "meigma/template-go-api/.github/workflows/release.yml",
+    signer: str = "meigma/template-go-api/.github/workflows/attest.yml",
     missing_checksum: str | None = None,
     checksum_override: tuple[str, str] | None = None,
     omit_artifact: tuple[str, str, str] | None = None,

diff --git a/.github/workflows/attest.yml b/.github/workflows/attest.yml
@@ -0,0 +1,73 @@
+# Reusable workflow that generates artifact PROVENANCE in isolation from the build
+# jobs that call it. A reusable workflow runs in its own execution context with its
+# own OIDC identity — the caller can inject neither steps nor secrets — so the
+# signing material is unreachable by the build's user-defined steps. That isolation
+# is the SLSA Build L3 requirement; GitHub artifact attestations generated in-job
+# are only L2. Provenance is still written to GitHub's attestation API, so
+# `gh attestation verify` keeps working — but the signer-workflow is now this file:
+#
+#   gh attestation verify <artifact-or-oci-ref> --repo <repo> \
+#     --signer-workflow <repo>/.github/workflows/attest.yml --source-ref refs/tags/<tag>
+#
+# SLSA note: L3 makes the signed provenance unforgeable; it does not, by itself,
+# make the build hermetic. The build job still computes the checksums/digest it
+# passes in — L3's guarantee is that the signature cannot be tampered with because
+# the key lives only in this isolated workflow.
+
+name: Attest (reusable)
+
+on:
+  workflow_call:
+    inputs:
+      checksums-artifact:
+        description: Name of an uploaded artifact containing checksums.txt (binary provenance).
+        type: string
+        required: false
+        default: ''
+      subject-name:
+        description: Fully-qualified image name without tag/digest (container provenance).
+        type: string
+        required: false
+        default: ''
+      subject-digest:
+        description: Image digest in sha256:... form (container provenance).
+        type: string
+        required: false
+        default: ''
+      push-to-registry:
+        description: Attach the image provenance attestation to the registry.
+        type: boolean
+        required: false
+        default: false
+
+permissions: {}
+
+jobs:
+  attest:
+    name: Attest
+    runs-on: ubuntu-24.04
+    permissions:
+      id-token: write       # OIDC token minted HERE, isolated from the build job
+      attestations: write   # write provenance to GitHub's attestation API
+      contents: read
+      packages: write       # used only for the image attestation (push-to-registry)
+    steps:
+      - name: Download checksums
+        if: ${{ inputs.checksums-artifact != '' }}
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        with:
+          name: ${{ inputs.checksums-artifact }}
+
+      - name: Attest binary checksums
+        if: ${{ inputs.checksums-artifact != '' }}
+        uses: actions/attest@59d89421af93a897026c735860bf21b6eb4f7b26 # v4.1.0
+        with:
+          subject-checksums: checksums.txt
+
+      - name: Attest image provenance
+        if: ${{ inputs.subject-digest != '' }}
+        uses: actions/attest-build-provenance@0f67c3f4856b2e3261c31976d6725780e5e4c373 # v4.1.1
+        with:
+          subject-name: ${{ inputs.subject-name }}
+          subject-digest: ${{ inputs.subject-digest }}
+          push-to-registry: ${{ inputs.push-to-registry }}
diff --git a/.github/workflows/release-dry-run.yml b/.github/workflows/release-dry-run.yml
@@ -62,7 +62,7 @@ jobs:
           version="0.0.0-dryrun.${GITHUB_RUN_ID}.${GITHUB_RUN_ATTEMPT}"
           binary_name="template-go-api"
           asset_prefix="${binary_name}_"
-          expected_signer="${GITHUB_REPOSITORY}/.github/workflows/release.yml"
+          expected_signer="${GITHUB_REPOSITORY}/.github/workflows/attest.yml"
           actual_signer="$(awk -F'"' '/signer_workflow/ { print $2; exit }' ghd.toml)"
 
           if [ "$actual_signer" != "$expected_signer" ]; then

diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
@@ -78,8 +78,6 @@ jobs:
       - resolve-release
     permissions:
       contents: write
-      id-token: write
-      attestations: write
     steps:
       - name: Check out repository
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
@@ -148,10 +146,26 @@ jobs:
           RELEASE_TAG: ${{ needs.resolve-release.outputs.tag }}
         run: gh release upload "$RELEASE_TAG" dist/release-assets/* --clobber
 
-      - name: Attest release checksums
-        uses: actions/attest@59d89421af93a897026c735860bf21b6eb4f7b26 # v4.1.0
+      - name: Upload checksums for isolated attestation
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          subject-checksums: dist/release-assets/checksums.txt
+          name: release-checksums
+          path: dist/release-assets/checksums.txt
+          if-no-files-found: error
+          retention-days: 1
+
+  # Generate binary provenance in an ISOLATED reusable workflow (SLSA L3): the
+  # signing OIDC token is minted in attest.yml, unreachable by the build steps.
+  attest-binaries:
+    needs:
+      - binary-release-assets
+    permissions:
+      id-token: write
+      attestations: write
+      contents: read
+    uses: ./.github/workflows/attest.yml
+    with:
+      checksums-artifact: release-checksums
 
   # Build the per-arch signed apk on a native runner (no QEMU). Each runner mints
   # its own ephemeral melange key (distinct filename) and uploads its apk plus its
@@ -346,13 +360,6 @@ jobs:
           IMAGE_REF: ${{ steps.publish.outputs.ref }}
         run: cosign sign --yes "$IMAGE_REF"
 
-      - name: Attest container image provenance
-        uses: actions/attest-build-provenance@0f67c3f4856b2e3261c31976d6725780e5e4c373 # v4.1.1
-        with:
-          subject-name: ${{ steps.publish.outputs.name }}
-          subject-digest: ${{ steps.publish.outputs.digest }}
-          push-to-registry: true
-
       - name: Generate image SBOM (Go module granularity)
         env:
           IMAGE_REF: ${{ steps.publish.outputs.ref }}
@@ -366,13 +373,30 @@ jobs:
           sbom-path: image.spdx.json
           push-to-registry: true
 
+  # Generate container provenance in the ISOLATED reusable workflow (SLSA L3).
+  attest-image:
+    needs:
+      - container-image-release
+    permissions:
+      id-token: write
+      attestations: write
+      packages: write
+      contents: read
+    uses: ./.github/workflows/attest.yml
+    with:
+      subject-name: ${{ needs.container-image-release.outputs.image-name }}
+      subject-digest: ${{ needs.container-image-release.outputs.image-digest }}
+      push-to-registry: true
+
   release-inspection-summary:
     name: Release Inspection Summary
     runs-on: ubuntu-24.04
     needs:
       - resolve-release
       - binary-release-assets
       - container-image-release
+      - attest-binaries
+      - attest-image
     permissions: {}
     steps:
       - name: Write inspection summary
@@ -392,7 +416,7 @@ jobs:
             echo '```sh'
             echo "gh release view $RELEASE_TAG --repo $GITHUB_REPOSITORY --json isDraft,assets"
             echo "asset=\"template-go-api_${RELEASE_VERSION}_\$(go env GOOS)_\$(go env GOARCH)\""
-            echo "gh attestation verify \"dist/release-assets/\${asset}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/release.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
+            echo "gh attestation verify \"dist/release-assets/\${asset}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/attest.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
             echo "ghd download \"$GITHUB_REPOSITORY/template-go-api@${RELEASE_VERSION}\" --output \"\$(mktemp -d)\""
             echo '```'
             echo
@@ -402,7 +426,7 @@ jobs:
             echo "docker login ghcr.io"
             echo "docker pull \"${IMAGE_NAME}:${RELEASE_TAG}\""
             echo "docker run --rm \"${IMAGE_NAME}:${RELEASE_TAG}\" --version"
-            echo "gh attestation verify \"oci://${IMAGE_NAME}@${IMAGE_DIGEST}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/release.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
+            echo "gh attestation verify \"oci://${IMAGE_NAME}@${IMAGE_DIGEST}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/attest.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
             echo "cosign verify \"${IMAGE_NAME}@${IMAGE_DIGEST}\" --certificate-identity-regexp \"^https://github.com/${GITHUB_REPOSITORY}/.github/workflows/release.yml@.*\" --certificate-oidc-issuer https://token.actions.githubusercontent.com"
             echo '```'
             echo

diff --git a/DELETE_ME.md b/DELETE_ME.md
@@ -155,7 +155,7 @@ The nominal generated-project path is an HTTP service with both a downloadable b
    For library-only projects:
 
    - Keep Release Please if version tags and changelogs are useful.
-   - Delete `.github/workflows/release.yml`, `.github/workflows/release-dry-run.yml`, `.github/workflows/security-scan.yml`, `.goreleaser.yaml`, `ghd.toml`, `melange.yaml`, and `apko.yaml` unless the library publishes some other artifact.
+   - Delete `.github/workflows/release.yml`, `.github/workflows/release-dry-run.yml`, `.github/workflows/attest.yml`, `.github/workflows/security-scan.yml`, `.goreleaser.yaml`, `ghd.toml`, `melange.yaml`, and `apko.yaml` unless the library publishes some other artifact.
    - Remove release dry-run checks from `.github/repository-settings.toml`.
    - If the library should not create releases at all, delete `.github/workflows/release-please.yml`, `release-please-config.json`, `.release-please-manifest.json`, and `CHANGELOG.md`.
 

diff --git a/README.md b/README.md
@@ -706,10 +706,11 @@ The release path is:
 
 - Release Please opens and maintains the release PR.
 - Release Please creates a draft GitHub release and tag after merge.
-- Release Dry Run rehearses the GoReleaser binary path and native-runner Docker container build path on pull requests.
+- Release Dry Run rehearses the GoReleaser binary path and the native-runner melange/apko container build path on pull requests.
 - GoReleaser builds binaries, checksums, and SBOMs without publishing directly.
-- The release workflow uploads assets to the draft release and creates a GitHub-hosted attestation for `checksums.txt`.
-- The release workflow builds amd64 and arm64 container images on native GitHub-hosted runners, publishes `ghcr.io/meigma/template-go-api:vX.Y.Z` as a multi-platform manifest, attaches BuildKit provenance and SBOM metadata, and creates a GitHub-native attestation for the manifest digest.
+- The release workflow uploads assets to the draft release; a separate, isolated reusable workflow (`attest.yml`) generates the GitHub-hosted provenance attestation for the binary checksums.
+- The release workflow builds amd64 and arm64 apks with melange on native GitHub-hosted runners, assembles and publishes `ghcr.io/meigma/template-go-api:vX.Y.Z` as a multi-platform manifest with apko, signs it with keyless cosign, and attaches a syft SBOM attestation; the isolated `attest.yml` workflow then creates the GitHub-native provenance attestation for the manifest digest.
+- Generating both provenance attestations in the isolated `attest.yml` reusable workflow (not in the build job) keeps the signing identity unreachable by build steps — the SLSA Build L3 isolation requirement — while staying on GitHub's attestation API (verify with `gh attestation verify --signer-workflow …/attest.yml`).
 - A human inspects the draft release before publication.
 
 The root `ghd.toml` matches the default GoReleaser output so generated projects can be installed with `ghd` once the release workflow runs.

diff --git a/ghd.toml b/ghd.toml
@@ -1,7 +1,7 @@
 version = 1
 
 [provenance]
-signer_workflow = "meigma/template-go-api/.github/workflows/release.yml"
+signer_workflow = "meigma/template-go-api/.github/workflows/attest.yml"
 
 [[packages]]
 name = "template-go-api"