Attest the build provenance of every release file

.github/workflows/release.yml

@@ -12,6 +12,8 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: write
+      id-token: write # to sign the attestation with Sigstore
+      attestations: write # to store the attestation with GitHub
     steps:
       # GoReleaser skips signing when MACOS_SIGN_P12 is empty, and a missing
       # secret arrives as an empty string. Fail rather than publish unsigned
@@ -78,3 +80,13 @@ jobs:
           MACOS_NOTARY_KEY_ID: ${{ secrets.MACOS_NOTARY_KEY_ID }}
           MACOS_NOTARY_KEY: ${{ secrets.MACOS_NOTARY_KEY }}
           HOMEBREW_TAP_TOKEN: ${{ secrets.HOMEBREW_TAP_TOKEN }}
+
+      # Sign a SLSA build provenance attestation for every file in
+      # SHA256SUMS, each binary and its SBOM: it records that this workflow
+      # built the file from the tagged commit. "gh attestation verify"
+      # checks it; see the README. The release is already published by now,
+      # so if this step fails, delete the GitHub release (not the tag) and
+      # re-run the workflow.
+      - uses: actions/attest@1e69f48acb82d1966a394da916b4c1698aa569d6 # v4.2.2
+        with:
+          subject-checksums: dist/SHA256SUMS

README.md

@@ -45,6 +45,24 @@ that `go install` records. To check a release or `make dist` output, run
 (`shasum -a 256 -c --ignore-missing SHA256SUMS` on macOS) in the directory
 that holds it.
 
+### Verify a release
+
+From v0.2.0 on, every release file has a signed build provenance
+attestation, which shows that this repository's release workflow built it
+from the tagged commit. A checksum only shows that a file matches
+`SHA256SUMS`, which is published beside it; the attestation shows where both
+came from. With the [GitHub CLI](https://cli.github.com):
+
+```sh
+gh attestation verify rypt-linux-amd64 \
+  --repo x6c-co/rypt-cli \
+  --signer-workflow x6c-co/rypt-cli/.github/workflows/release.yml
+```
+
+The same command checks the `rypt` that Homebrew installed: give it
+`"$(command -v rypt)"` instead of a file name. Each binary's SBOM
+(`rypt-linux-amd64.cdx.json` and so on) is attested the same way.
+
 ## Usage
 
 ```

docs/releasing.md

@@ -28,7 +28,10 @@ so on.
 Each binary has a CycloneDX SBOM beside it, named like it
 (`rypt-linux-amd64.cdx.json`), which GoReleaser makes with Syft from the
 binary's own build information: the Go version and every module compiled in.
-The SBOMs are listed in `SHA256SUMS` too. The release workflow builds with
+The SBOMs are listed in `SHA256SUMS` too, and the workflow's last step signs a
+build provenance attestation for every file in `SHA256SUMS` with
+[actions/attest](https://github.com/actions/attest); the README shows how to
+verify one. The release workflow builds with
 the newest Go release, whatever go.mod's `go` line says, so that binaries
 get the standard library's security fixes.