Sign checksum with cosign

Adds config to sign artifacts. Since the checksum contains the SHAs of
the artifacts, signing the checksums is enough to ensure that the artifacts
were not modified.

GoReleaser uses cosign to sign the artifact and uploads .sig and .pem to
the release.

.github/workflows/ci.yml

@@ -87,6 +87,7 @@ jobs:
       release-url: ${{ steps.release-notes.outputs.release-url }}
     permissions:
       contents: write # for lucacome/draft-release and goreleaser/goreleaser-action to manage releases
+      id-token: write # for goreleaser/goreleaser-action to sign artifacts
     steps:
       - name: Checkout Repository
         uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9 # v3.5.3
@@ -100,7 +101,11 @@ jobs:
 
       - name: Download Syft
         uses: anchore/sbom-action/download-syft@78fc58e266e87a38d4194b2137a3d4e9bcaf7ca1 # v0.14.3
-        if: startsWith(github.ref, 'refs/tags/')
+        if: github.ref_type == 'tag'
+
+      - name: Install Cosign
+        uses: sigstore/cosign-installer@6e04d228eb30da1757ee4e1dd75a0ec73a653e06 # v3.1.1
+        if: github.ref_type == 'tag'
 
       - name: Create/Update Draft
         uses: lucacome/draft-release@f6dc37dcdf44be100a649b72c62c628776750190 # v0.2.2

.goreleaser.yml

@@ -117,6 +117,18 @@ blobs:
     extra_files:
       - glob: ./dist/**.spdx.json
 
+signs:
+  - cmd: cosign
+    artifacts: checksum
+    output: true
+    certificate: '${artifact}.pem'
+    args:
+      - sign-blob
+      - "--output-signature=${signature}"
+      - "--output-certificate=${certificate}"
+      - "${artifact}"
+      - "--yes"
+
 announce:
   slack:
     enabled: true