Add image signing with GPG tutorial

Signed-off-by: Sascha Grunert <sgrunert@suse.com>

docs/tutorials/README.md

@@ -23,3 +23,7 @@ A brief how-to on using the Podman remote-client.
 **[How to use libpod for custom/derivative projects](podman-derivative-api.md)**
 
 How the libpod API can be used within your own project.
+
+**[Image Signing](image_signing.md)**
+
+Learn how to setup and use image signing with Podman.

docs/tutorials/image_signing.md

@@ -0,0 +1,194 @@
+# How to sign and distribute container images using Podman
+
+Signing container images originates from the motivation of trusting only
+dedicated image providers to mitigate man-in-the-middle (MITM) attacks or
+attacks on container registries. One way to sign images is to utilize a GNU
+Privacy Guard ([GPG][0]) key. This technique is generally compatible with any
+OCI compliant container registry like [Quay.io][1]. It is worth mentioning that
+the OpenShift integrated container registry supports this signing mechanism out
+of the box, which makes separate signature storage unnecessary.
+
+[0]: https://gnupg.org
+[1]: https://quay.io
+
+From a technical perspective, we can utilize Podman to sign the image before
+pushing it into a remote registry. After that, all systems running Podman have
+to be configured to retrieve the signatures from a remote server, which can
+be any simple web server. This means that every unsigned image will be rejected
+during an image pull operation. But how does this work?
+
+First of all, we have to create a GPG key pair or select an already locally
+available one. To generate a new GPG key, just run `gpg --full-gen-key` and
+follow the interactive dialog. Now we should be able to verify that the key
+exists locally:
+
+```bash
+> gpg --list-keys sgrunert@suse.com
+pub   rsa2048 2018-11-26 [SC] [expires: 2020-11-25]
+      XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+uid           [ultimate] Sascha Grunert <sgrunert@suse.com>
+sub   rsa2048 2018-11-26 [E] [expires: 2020-11-25]
+```
+
+Now let’s assume that we run a container registry. For example we could simply
+start one on our local machine:
+
+```bash
+> sudo podman run -d -p 5000:5000 docker.io/registry
+```
+
+The registry does not know anything about image signing, it just provides the remote
+storage for the container images. This means if we want to sign an image, we
+have to take care of how to distribute the signatures.
+
+Let’s choose a standard `alpine` image for our signing experiment:
+
+```bash
+> sudo podman pull docker://docker.io/alpine:latest
+```
+
+```bash
+> sudo podman images alpine
+REPOSITORY                 TAG      IMAGE ID       CREATED       SIZE
+docker.io/library/alpine   latest   e7d92cdc71fe   6 weeks ago   5.86 MB
+```
+
+Now we can re-tag the image to point it to our local registry:
+
+```bash
+> sudo podman tag alpine localhost:5000/alpine
+```
+
+```bash
+> sudo podman images alpine
+REPOSITORY                 TAG      IMAGE ID       CREATED       SIZE
+localhost:5000/alpine      latest   e7d92cdc71fe   6 weeks ago   5.86 MB
+docker.io/library/alpine   latest   e7d92cdc71fe   6 weeks ago   5.86 MB
+```
+
+Podman would now be able to push the image and sign it in one command. But to
+let this work, we have to modify our system-wide registries configuration at
+`/etc/containers/registries.d/default.yaml`:
+
+```yaml
+default-docker:
+  sigstore: http://localhost:8000 # Added by us
+  sigstore-staging: file:///var/lib/containers/sigstore
+```
+
+We can see that we have two signature stores configured:
+
+- `sigstore`: referencing a web server for signature reading
+- `sigstore-staging`: referencing a file path for signature writing
+
+Now, let’s push and sign the image:
+
+```bash
+> sudo -E GNUPGHOME=$HOME/.gnupg \
+    podman push \
+    --tls-verify=false \
+    --sign-by sgrunert@suse.com \
+    localhost:5000/alpine
+…
+Storing signatures
+```
+
+If we now take a look at the systems signature storage, then we see that there
+is a new signature available, which was caused by the image push:
+
+```bash
+> sudo ls /var/lib/containers/sigstore
+'alpine@sha256=e9b65ef660a3ff91d28cc50eba84f21798a6c5c39b4dd165047db49e84ae1fb9'
+```
+
+The default signature store in our edited version of
+`/etc/containers/registries.d/default.yaml` references a web server listening at
+`http://localhost:8000`. For our experiment, we simply start a new server inside
+the local staging signature store:
+
+```bash
+> sudo bash -c 'cd /var/lib/containers/sigstore && python3 -m http.server'
+Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...
+```
+
+Let’s remove the local images for our verification test:
+
+```
+> sudo podman rmi docker.io/alpine localhost:5000/alpine
+```
+
+We have to write a policy to enforce that the signature has to be valid. This
+can be done by adding a new rule in `/etc/containers/policy.json`. From the
+below example, copy the `"docker"` entry into the `"transports"` section of your
+`policy.json`.
+
+```json
+{
+  "default": [{ "type": "insecureAcceptAnything" }],
+  "transports": {
+    "docker": {
+      "localhost:5000": [
+        {
+          "type": "signedBy",
+          "keyType": "GPGKeys",
+          "keyPath": "/tmp/key.gpg"
+        }
+      ]
+    }
+  }
+}
+```
+
+The `keyPath` does not exist yet, so we have to put the GPG key there:
+
+```bash
+> gpg --output /tmp/key.gpg --armor --export sgrunert@suse.com
+```
+
+If we now pull the image:
+
+```bash
+> sudo podman pull --tls-verify=false localhost:5000/alpine
+…
+Storing signatures
+e7d92cdc71feacf90708cb59182d0df1b911f8ae022d29e8e95d75ca6a99776a
+```
+
+Then we can see in the logs of the web server that the signature has been
+accessed:
+
+```
+127.0.0.1 - - [04/Mar/2020 11:18:21] "GET /alpine@sha256=e9b65ef660a3ff91d28cc50eba84f21798a6c5c39b4dd165047db49e84ae1fb9/signature-1 HTTP/1.1" 200 -
+```
+
+As an counterpart example, if we specify the wrong key at `/tmp/key.gpg`:
+
+```bash
+> gpg --output /tmp/key.gpg --armor --export mail@saschagrunert.de
+File '/tmp/key.gpg' exists. Overwrite? (y/N) y
+```
+
+Then a pull is not possible any more:
+
+```bash
+> sudo podman pull --tls-verify=false localhost:5000/alpine
+Trying to pull localhost:5000/alpine...
+Error: error pulling image "localhost:5000/alpine": unable to pull localhost:5000/alpine: unable to pull image: Source image rejected: Invalid GPG signature: …
+```
+
+So in general there are four main things to be taken into consideration when
+signing container images with Podman and GPG:
+
+1. We need a valid private GPG key on the signing machine and corresponding
+   public keys on every system which would pull the image
+2. A web server has to run somewhere which has access to the signature storage
+3. The web server has to be configured in any
+   `/etc/containers/registries.d/*.yaml` file
+4. Every image pulling system has to be configured to contain the enforcing
+   policy configuration via `policy.conf`
+
+That’s it for image signing and GPG. The cool thing is that this setup works out
+of the box with [CRI-O][2] as well and can be used to sign container images in
+Kubernetes environments.
+
+[2]: https://cri-o.io