Switch from GPG to Sigstore for Python source verification

The build scripts used to download and compile the Python source
archives (for upload to S3, where they are then consumed by the
buildpack during customer builds) currently use GPG to verify the Python
source archive downloads.

However, use of PGP signatures for Python artifact verification was
deprecated previously in PEP 761, in favour of Sigstore:
https://peps.python.org/pep-0761/
https://www.python.org/downloads/metadata/sigstore/

Until now the PGP signatures have still been available for all stable
releases, however, as of Python 3.14 (due to be released this week),
Sigstore will be the only supported verification mechanism:
https://docs.python.org/3.14/whatsnew/3.14.html#whatsnew314-no-more-pgp

As such, we must now switch over to Sigstore.

We use the `cosign` CLI for verification since it's a standalone binary
available via a Docker image, rather than the Python `sigstore` CLI
which requires a Python environment (and so pip, venv etc, and more
setup to ensure it stays isolated from the Python we're trying to
build).

See:
- https://www.python.org/downloads/metadata/sigstore/
- https://docs.sigstore.dev/cosign/system_config/installation/#container-images
- https://docs.sigstore.dev/cosign/verifying/verify/
- https://github.com/sigstore/cosign/blob/main/doc/cosign_verify-blob.md

GUS-W-18244071.

Author: edmorley

.github/dependabot.yml

@@ -13,6 +13,14 @@ updates:
         update-types:
           - "minor"
           - "patch"
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "dependencies"
+      - "docker"
+      - "skip changelog"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:

builds/Dockerfile

@@ -1,4 +1,5 @@
 ARG STACK_VERSION="24"
+FROM ghcr.io/sigstore/cosign/cosign:v2.6.1@sha256:68839b7f13dac5a6744a5d8818e984dd39183374e37855c19e14d623d9bc9037 AS cosign
 FROM heroku/heroku:${STACK_VERSION}-build
 
 ARG STACK_VERSION
@@ -13,6 +14,7 @@ RUN apt-get update --error-on=any \
       libreadline-dev \
       libsqlite3-dev \
     && rm -rf /var/lib/apt/lists/*
+COPY --from=cosign /ko-app/cosign /usr/local/bin/cosign
 
 WORKDIR /tmp
 COPY build_python_runtime.sh python-3.13-ubuntu-22.04-libexpat-workaround.patch .

builds/build_python_runtime.sh

@@ -43,23 +43,19 @@ if [[ " ${SUPPORTED_PYTHON_VERSIONS[*]} " != *" ${PYTHON_MAJOR_VERSION} "* ]]; t
 	abort "Python ${PYTHON_MAJOR_VERSION} isn't supported on ${STACK}!"
 fi
 
-# The release keys can be found on https://www.python.org/downloads/ -> "OpenPGP Public Keys".
+# Sigstore identities taken from: https://www.python.org/downloads/metadata/sigstore/
 case "${PYTHON_MAJOR_VERSION}" in
-	3.13)
-		# https://github.com/Yhg1s.gpg
-		GPG_KEY_FINGERPRINT='7169605F62C751356D054A26A821E680E5FA6305'
-		;;
-	3.12)
-		# https://github.com/Yhg1s.gpg
-		GPG_KEY_FINGERPRINT='7169605F62C751356D054A26A821E680E5FA6305'
+	3.12 | 3.13)
+		SIGSTORE_IDENTITY='thomas@python.org'
+		SIGSTORE_ISSUER='https://accounts.google.com'
 		;;
 	3.10 | 3.11)
-		# https://keybase.io/pablogsal/
-		GPG_KEY_FINGERPRINT='A035C8C19219BA821ECEA86B64E628F8D684696D'
+		SIGSTORE_IDENTITY='pablogsal@python.org'
+		SIGSTORE_ISSUER='https://accounts.google.com'
 		;;
 	3.9)
-		# https://keybase.io/ambv/
-		GPG_KEY_FINGERPRINT='E3FF2839C048B25C084DEBE9B26995E310250568'
+		SIGSTORE_IDENTITY='lukasz@langa.pl'
+		SIGSTORE_ISSUER='https://github.com/login/oauth'
 		;;
 	*)
 		abort "Unsupported Python version '${PYTHON_MAJOR_VERSION}'!"
@@ -69,17 +65,20 @@ esac
 echo "Building Python ${PYTHON_VERSION} for ${STACK} (${ARCH})..."
 
 SOURCE_URL="https://www.python.org/ftp/python/${PYTHON_VERSION}/Python-${PYTHON_VERSION}.tgz"
-SIGNATURE_URL="${SOURCE_URL}.asc"
+SIGSTORE_BUNDLE_URL="${SOURCE_URL}.sigstore"
 
 set -o xtrace
 
 mkdir -p "${SRC_DIR}" "${INSTALL_DIR}" "${UPLOAD_DIR}"
 
 curl --fail --retry 5 --retry-connrefused --connect-timeout 3 --max-time 30 -o python.tgz "${SOURCE_URL}"
-curl --fail --retry 5 --retry-connrefused --connect-timeout 3 --max-time 30 -o python.tgz.asc "${SIGNATURE_URL}"
+curl --fail --retry 5 --retry-connrefused --connect-timeout 3 --max-time 30 -o python.tgz.sigstore "${SIGSTORE_BUNDLE_URL}"
 
-gpg --batch --verbose --recv-keys "${GPG_KEY_FINGERPRINT}"
-gpg --batch --verify python.tgz.asc python.tgz
+cosign verify-blob \
+	--bundle python.tgz.sigstore \
+	--certificate-identity "${SIGSTORE_IDENTITY}" \
+	--certificate-oidc-issuer "${SIGSTORE_ISSUER}" \
+	python.tgz
 
 tar --extract --file python.tgz --strip-components=1 --directory "${SRC_DIR}"
 cd "${SRC_DIR}"