Sign Docker images and record max-mode provenance

Dockerfile

@@ -52,6 +52,8 @@ RUN mkdir -p /usr/libexec/docker/cli-plugins && \
       -o /usr/libexec/docker/cli-plugins/docker-buildx && \
     chmod +x /usr/libexec/docker/cli-plugins/docker-buildx
 
+COPY --from=ghcr.io/sigstore/cosign/cosign:v3.1.3@sha256:9e5c2f2edc34351160407ca3416c61855bdf9403c3c5936e0f0be7fc261611b8 /ko-app/cosign /usr/local/bin/cosign
+
 RUN echo "deb [signed-by=/usr/share/keyrings/cloud.google.gpg] https://packages.cloud.google.com/apt cloud-sdk main" | tee -a /etc/apt/sources.list.d/google-cloud-sdk.list && \
     curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | gpg --dearmor -o /usr/share/keyrings/cloud.google.gpg && \
     apt update -y && apt install --no-install-recommends -y google-cloud-cli

README.md

@@ -80,3 +80,20 @@ Requesting an image also reserves it from cleanup, permanently. If the image alr
 Cleanup only touches `hexpm/elixir-amd64` and `hexpm/elixir-arm64`, which hold the single-arch images the multi-arch manifests are assembled from rather than images meant to be pulled directly. They keep the last 30 days of builds; older single-arch tags are removed, and the multi-arch image they back stays available.
 
 Tags reserved by a build request are never removed and are flagged as `reserved`. To pin a single-arch tag indefinitely, request it at https://bob.hex.pm/request.
+
+### Verifying images
+
+Images are signed with [cosign](https://github.com/sigstore/cosign) using a key held in Google Cloud KMS, and every signature is recorded in the public [Rekor](https://docs.sigstore.dev/logging/overview/) transparency log. The public key is [`cosign.pub`](cosign.pub). Verify an image with:
+
+```
+curl -fsSLO https://raw.githubusercontent.com/hexpm/bob/main/cosign.pub
+cosign verify --key cosign.pub hexpm/elixir:<tag>
+```
+
+Tags built before signing was added have no signature.
+
+Each image also carries a [SLSA provenance](https://docs.docker.com/build/metadata/attestations/slsa-provenance/) attestation recording the build arguments, the Dockerfile and the digests of the base images it was built from:
+
+```
+docker buildx imagetools inspect hexpm/elixir:<tag> --format '{{json .Provenance}}'
+```

cosign.pub

@@ -0,0 +1,4 @@
+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEWV4rz84tD9WropTPo3OaJPGhNw7+
+4klLaSVMgijgDryNV6JBWG3ZEZ1x6qo2XPMeo9blVblN0NX1LoRQtkXP1Q==
+-----END PUBLIC KEY-----

priv/scripts/docker/elixir.sh

@@ -11,7 +11,7 @@ arch=$5
 tag=${elixir}-erlang-${erlang}-${os}-${os_version}
 erlang_major=$(echo "${erlang}" | awk 'match($0, /^[0-9][0-9]/) { print substr( $0, RSTART, RLENGTH )}')
 
-docker build --ulimit nofile=1024:1024 -t hexpm/elixir-${arch}:${tag} --build-arg ELIXIR=${elixir} --build-arg ERLANG=${erlang} --build-arg ERLANG_MAJOR=${erlang_major} --build-arg OS_VERSION=${os_version} --build-arg ARCH=${arch} -f ${SCRIPT_DIR}/docker/elixir-${os}.dockerfile ${SCRIPT_DIR}/docker
+docker build --ulimit nofile=1024:1024 --provenance=mode=max -t hexpm/elixir-${arch}:${tag} --build-arg ELIXIR=${elixir} --build-arg ERLANG=${erlang} --build-arg ERLANG_MAJOR=${erlang_major} --build-arg OS_VERSION=${os_version} --build-arg ARCH=${arch} -f ${SCRIPT_DIR}/docker/elixir-${os}.dockerfile ${SCRIPT_DIR}/docker
 
 # Smoke test
 docker run --rm hexpm/elixir-${arch}:${tag} elixir -v
@@ -23,3 +23,5 @@ docker push docker.io/hexpm/elixir-${arch}:${tag} ||
   (sleep $((20 + $RANDOM % 40)) && docker push docker.io/hexpm/elixir-${arch}:${tag}) ||
   (sleep $((20 + $RANDOM % 40)) && docker push docker.io/hexpm/elixir-${arch}:${tag}) ||
   (exit 1)
+
+${SCRIPT_DIR}/docker/sign.sh hexpm/elixir-${arch} ${tag}

priv/scripts/docker/erlang.sh

@@ -39,6 +39,7 @@ fi
 
 docker build \
   --ulimit nofile=1024:1024 \
+  --provenance=mode=max \
   -t hexpm/erlang-${arch}:${tag} \
   --build-arg ARCH=${arch} \
   --build-arg ERLANG=${erlang} \
@@ -58,3 +59,5 @@ docker push docker.io/hexpm/erlang-${arch}:${tag} ||
   (sleep $((20 + $RANDOM % 40)) && docker push docker.io/hexpm/erlang-${arch}:${tag}) ||
   (sleep $((20 + $RANDOM % 40)) && docker push docker.io/hexpm/erlang-${arch}:${tag}) ||
   (exit 1)
+
+${SCRIPT_DIR}/docker/sign.sh hexpm/erlang-${arch} ${tag}

priv/scripts/docker/manifest.sh

@@ -20,3 +20,5 @@ docker buildx imagetools create -t hexpm/${kind}:${tag} ${arch_images} ||
   (sleep $((10 + $RANDOM % 20)) && docker buildx imagetools create -t hexpm/${kind}:${tag} ${arch_images}) ||
   (sleep $((10 + $RANDOM % 20)) && docker buildx imagetools create -t hexpm/${kind}:${tag} ${arch_images}) ||
   (exit 1)
+
+${SCRIPT_DIR}/docker/sign.sh hexpm/${kind} ${tag}

priv/scripts/docker/sign.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+set -euox pipefail
+
+repo=$1
+tag=$2
+
+# Sign the digest that was pushed, since the tag can be moved
+digest=sha256:$(docker buildx imagetools inspect --raw ${repo}:${tag} | sha256sum | cut -d ' ' -f 1)
+
+# Cloud KMS is called as the service account whose key Bob.Application writes here
+export GOOGLE_APPLICATION_CREDENTIALS=/boto/keyfile.json
+
+# This command has a tendency to intermittently fail
+cosign sign --yes --key ${BOB_COSIGN_KEY} ${repo}@${digest} ||
+  (sleep $((20 + $RANDOM % 40)) && cosign sign --yes --key ${BOB_COSIGN_KEY} ${repo}@${digest}) ||
+  (sleep $((20 + $RANDOM % 40)) && cosign sign --yes --key ${BOB_COSIGN_KEY} ${repo}@${digest}) ||
+  (sleep $((20 + $RANDOM % 40)) && cosign sign --yes --key ${BOB_COSIGN_KEY} ${repo}@${digest}) ||
+  (sleep $((20 + $RANDOM % 40)) && cosign sign --yes --key ${BOB_COSIGN_KEY} ${repo}@${digest}) ||
+  (exit 1)