Docker Content Trust "offline" possible/reasonable? - docker

Question

I'm currently faced with the task of signing our internally created docker images stored in an Artifactory docker repository on premise.
We have a target environment which (currently) has no access to the internet nor to our internal docker registry.
I've learned so far, that by enabling docker content trust with
export DOCKER_CONTENT_TRUST=1
on the machine building the images is mandatory. As far as I understood the documentation the procedure is:
Enable docker content trust on the build client
use docker push which will generate the root and targets key
Store the key(s) in a save location
Upload the image to artifactory
Is it correct, that with 2. the official notary server is/must be used to verify, that the image is indeed signed by our company?
I'm just wondering if our current deployment scenario can use docker content trust:
Store image as myDockerImage.tar.gz (i.e. docker save <IMAGE_NAME>)
copy tar.gz file to target machine
use docker load -i <FILENAME>.tar.gz to import image to local registry on the target machine
docker run (< Must fail if image is not signed by our key)
As already stated the target machine neither has access to our infrastructure nor to the internet. Is it advisable to use docker content trust for this "offline" scenario? Is there a keyfile that can be put on the target machine instead of having a connection to the notary server?

Answer 1

After digging for a while I came to the conclusion, that using
shasum256 <FILE(S)> deployment-files.sha256
in combination with a signature using which signs the deployment-files.sha256 via openssl is my best option.

Answer 2

For totally offline you can't rely on docker trust
You can associate the image repo digest (the sha256 you can use during docker pull) with the digest of a saved image file:
#!/bin/sh
set -e
IMAGE_TAG=alpine:3.14
IMAGE_HASH=sha256:a573d30bfc94d672abd141b3bf320b356e731e3b1a7d79a8ab46ba65c11d79e1
docker pull ${IMAGE_TAG}#${IMAGE_HASH}
# save the image id so the recipient can find the image after docker load
IMAGE_ID=$(docker image inspect ${IMAGE_TAG}#${IMAGE_HASH} -f {{.Id}})
echo ${IMAGE_ID} > ${IMAGE_TAG}.id
docker save -o ${IMAGE_TAG} ${IMAGE_TAG}#${IMAGE_HASH}
sha256sum ${IMAGE_TAG} ${IMAGE_TAG}.id > ${IMAGE_TAG}.sha256
You now have an image that can be used with docker load, and a digest for that image that you can verify at the receiver (assuming you trust how you convey the digest file to them - ie, you can sign it)
The recipient of the image files can then do something like:
sha256sum -c alpine:3.14.sha256
docker load -i alpine:3.14
docker inspect $(cat alpine:3.14.id)