fix(release): sign and attest GitHub release assets directly

.devcontainer/Dockerfile

@@ -58,6 +58,7 @@ RUN apt-get update \
 # ACTIONLINT_VERSION -> https://github.com/rhysd/actionlint/releases
 # HADOLINT_VERSION -> https://github.com/hadolint/hadolint/releases
 # VALKEY_VERSION -> https://github.com/valkey-io/valkey/releases
+# COSIGN_VERSION -> https://github.com/sigstore/cosign/releases
 
 ENV PATH="/go/bin:/usr/local/go/bin:${PATH}" \
     KUBECTL_VERSION=v1.36.2 \
@@ -71,7 +72,8 @@ ENV PATH="/go/bin:/usr/local/go/bin:${PATH}" \
     TASK_VERSION=v3.51.1 \
     ACTIONLINT_VERSION=1.7.12 \
     HADOLINT_VERSION=2.14.0 \
-    VALKEY_VERSION=9.1.0
+    VALKEY_VERSION=9.1.0 \
+    COSIGN_VERSION=v3.1.1
 
 # Fail early on unsupported architectures instead of producing a partial image.
 RUN test "$(dpkg --print-architecture)" = "amd64" \
@@ -158,6 +160,22 @@ RUN curl -fsSL "https://download.valkey.io/releases/valkey-${VALKEY_VERSION}-jam
     | tar -xzO "valkey-${VALKEY_VERSION}-jammy-x86_64/bin/valkey-cli" > /usr/local/bin/valkey-cli \
     && chmod +x /usr/local/bin/valkey-cli
 
+# Install cosign (Sigstore keyless signing/verification). Lives in the ci stage,
+# not dev-only: release.yml's publish-helm job runs *inside* this same image
+# (see `container:` in .github/workflows/release.yml) and signs install.yaml
+# with cosign — baking it in here means that job no longer needs its own
+# sigstore/cosign-installer step. Also lets you run `cosign verify`/`verify-blob`
+# locally against what release.yml publishes (see docs/ci-overview.md).
+RUN asset="cosign-linux-amd64" \
+    && base="https://github.com/sigstore/cosign/releases/download/${COSIGN_VERSION}" \
+    && tmpdir="$(mktemp -d)" \
+    && curl -fsSL "${base}/${asset}" -o "${tmpdir}/${asset}" \
+    && curl -fsSL "${base}/cosign_checksums.txt" -o "${tmpdir}/checksums.txt" \
+    && cd "${tmpdir}" \
+    && grep "  ${asset}$" checksums.txt | sha256sum -c - \
+    && install -m 0755 "${asset}" /usr/local/bin/cosign \
+    && rm -rf "${tmpdir}"
+
 # Set working directory
 WORKDIR /workspaces
 

.github/workflows/ci.yml

@@ -107,6 +107,7 @@ jobs:
             controller-gen --version
             k3d version
             docker --version
+            cosign version
             echo '✅ All CI container tools verified'
           "
 

.github/workflows/release.yml

@@ -142,6 +142,12 @@ jobs:
 
       - name: Install cosign
         uses: sigstore/cosign-installer@6f9f17788090df1f26f669e9d70d6ae9567deba6 # v4.1.2
+        with:
+          # Keep in sync with COSIGN_VERSION in .devcontainer/Dockerfile — this
+          # is the only other job that signs release artifacts (publish-helm),
+          # and it gets cosign from that image. Without this input the action
+          # defaults to its own (older) pinned cosign release.
+          cosign-release: v3.1.1
 
       - name: Sign the multi-arch image (cosign keyless)
         # One signature on the digest covers every tag pointing at it.
@@ -171,6 +177,23 @@ jobs:
           sbom-path: sbom.spdx.json
           push-to-registry: true
 
+      # The image itself is already signed + attested above, but OpenSSF
+      # Scorecard's Signed-Releases check only looks at the GitHub *release
+      # assets*, not the OCI registry. Sign and attest the SBOM asset directly
+      # so the release itself carries a signature (*.sigstore.json) and SLSA
+      # provenance (*.intoto.jsonl) next to it.
+      - name: Sign the SBOM asset (cosign keyless)
+        run: cosign sign-blob --bundle sbom.spdx.json.sigstore.json --yes sbom.spdx.json
+
+      - name: Attest SLSA provenance for the SBOM asset
+        id: attest-sbom-asset
+        uses: actions/attest-build-provenance@0f67c3f4856b2e3261c31976d6725780e5e4c373 # v4.1.1
+        with:
+          subject-path: sbom.spdx.json
+
+      - name: Rename provenance bundle to the Scorecard-recognized suffix
+        run: cp "${{ steps.attest-sbom-asset.outputs.bundle-path }}" sbom.spdx.json.intoto.jsonl
+
       - name: Update release info
         uses: softprops/action-gh-release@718ea10b132b3b2eba29c1007bb80653f286566b # v3.0.1
         with:
@@ -181,7 +204,10 @@ jobs:
           # it to published after all assets are attached.
           draft: true
           append_body: true
-          files: sbom.spdx.json
+          files: |
+            sbom.spdx.json
+            sbom.spdx.json.sigstore.json
+            sbom.spdx.json.intoto.jsonl
           body: |
             ## Installation
             ### Quick Install (Single YAML)
@@ -226,7 +252,8 @@ jobs:
     permissions:
       contents: write # upload install.yaml release asset
       packages: write
-      id-token: write # cosign keyless signing
+      id-token: write # cosign keyless signing + attestation OIDC
+      attestations: write
     container:
       image: ${{ needs.ci.outputs.ci-image }}
       credentials:
@@ -262,9 +289,8 @@ jobs:
           fi
           echo "digest=${DIGEST}" >> "$GITHUB_OUTPUT"
 
-      - name: Install cosign
-        uses: sigstore/cosign-installer@6f9f17788090df1f26f669e9d70d6ae9567deba6 # v4.1.2
-
+      # cosign ships baked into the ci-image this job runs in (see
+      # .devcontainer/Dockerfile) — no installer step needed here.
       - name: Log in to registry for cosign
         # `helm registry login` writes helm's own registry config; cosign reads
         # ~/.docker/config.json, so it needs its own login or the signature
@@ -277,6 +303,23 @@ jobs:
       - name: Sign the Helm chart (cosign keyless)
         run: cosign sign --yes "${CHART_REGISTRY}/gitops-reverser@${{ steps.chart.outputs.digest }}"
 
+      # The chart is already signed above via its OCI digest, but OpenSSF
+      # Scorecard's Signed-Releases check only looks at the GitHub *release
+      # assets*, not the OCI registry. Sign and attest install.yaml directly so
+      # the release carries a signature (*.sigstore.json) and SLSA provenance
+      # (*.intoto.jsonl) next to it.
+      - name: Sign install.yaml (cosign keyless)
+        run: cosign sign-blob --bundle install.yaml.sigstore.json --yes dist/install.yaml
+
+      - name: Attest SLSA provenance for install.yaml
+        id: attest-install-yaml
+        uses: actions/attest-build-provenance@0f67c3f4856b2e3261c31976d6725780e5e4c373 # v4.1.1
+        with:
+          subject-path: dist/install.yaml
+
+      - name: Rename provenance bundle to the Scorecard-recognized suffix
+        run: cp "${{ steps.attest-install-yaml.outputs.bundle-path }}" install.yaml.intoto.jsonl
+
       - name: Upload install.yaml as release asset
         uses: softprops/action-gh-release@718ea10b132b3b2eba29c1007bb80653f286566b # v3.0.1
         with:
@@ -285,7 +328,10 @@ jobs:
           # every asset is in place (immutable releases reject post-publish
           # uploads).
           draft: true
-          files: dist/install.yaml
+          files: |
+            dist/install.yaml
+            install.yaml.sigstore.json
+            install.yaml.intoto.jsonl
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 

docs/ci-overview.md

@@ -84,7 +84,7 @@ step merges those exact digests into a multi-arch manifest tagged with the semve
 | --- | --- | --- |
 | Multi-arch image (`linux/amd64`, `linux/arm64`) | `ghcr.io/configbutler/gitops-reverser` | cosign keyless signature, SLSA build provenance attestation, SPDX SBOM attestation |
 | Helm chart | `oci://ghcr.io/configbutler/charts/gitops-reverser` | cosign keyless signature |
-| `install.yaml` + `sbom.spdx.json` | GitHub release assets | part of the signed release |
+| `install.yaml`, `sbom.spdx.json` | GitHub release assets | each signed directly (`<asset>.sigstore.json`) and SLSA-attested (`<asset>.intoto.jsonl`), also uploaded as release assets |
 
 Verify an image (also embedded in every release's notes):
 
@@ -98,6 +98,20 @@ gh attestation verify oci://ghcr.io/configbutler/gitops-reverser:<version> \
   --repo ConfigButler/gitops-reverser
 ```
 
+Verify a release asset (e.g. `install.yaml`) from its downloaded `.sigstore.json` bundle:
+
+```bash
+cosign verify-blob \
+  --bundle install.yaml.sigstore.json \
+  --certificate-identity-regexp '^https://github.com/ConfigButler/gitops-reverser/\.github/workflows/release\.yml@refs/heads/main$' \
+  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
+  install.yaml
+```
+
+These per-asset signatures/attestations exist specifically so release assets carry their
+own verifiable integrity, independent of the OCI registry — the surface OpenSSF Scorecard's
+`Signed-Releases` check actually inspects.
+
 Signing is *keyless* (Sigstore): there is no private key to store or leak. The signature
 certifies "built by the `release.yml` workflow on `main` of this repository", issued via
 GitHub's OIDC identity and logged in the public Rekor transparency log.