Encrypted Images

Context

A user might want to bundle sensitive data on an OCI (Docker) image. The image layers should only be accessible within a Trusted Execution Environment (TEE).

The project provides the means to encrypt an image with a symmetric key that is released to the TEE only after successful verification and appraisal in a Remote Attestation process. CoCo infrastructure components within the TEE will transparently decrypt the image layers as they are pulled from a registry without exposing the decrypted data outside the boundaries of the TEE.

Instructions

The following steps require a functional CoCo installation on a Kubernetes cluster. A Key Broker Client (KBC) has to be configured for TEEs to be able to retrieve confidential secrets. We assume cc_kbc as a KBC for the CoCo project’s Key Broker Service (KBS) in the following instructions, but image encryption should work with other Key Broker implementations in a similar fashion.

Please ensure you have a recent version of Skopeo (v1.14.2+) installed locally.

Encrypt an image

We extend public image with secret data.

docker build -t unencrypted - <<EOF
FROM nginx:stable
RUN echo "something confidential" > /secret
EOF

The encryption key needs to be a 32 byte sequence and provided to the encryption step as base64-encoded string.

KEY_FILE="image_key"
head -c 32 /dev/urandom | openssl enc > "$KEY_FILE"
KEY_B64="$(base64 < $KEY_FILE)"

The key id is a generic resource descriptor used by the key broker to look up secrets in its storage. For KBS this is composed of three segments: $repository_name/$resource_type/$resource_tag

KEY_PATH="/default/image_key/nginx"
KEY_ID="kbs://${KEY_PATH}"

The image encryption logic is bundled and invoked in a container:

git clone https://github.com/confidential-containers/guest-components.git
cd guest-components
docker build -t coco-keyprovider -f ./attestation-agent/docker/Dockerfile.keyprovider .

To access the image from within the container, Skopeo can be used to buffer the image in a directory, which is then made available to the container. Similarly, the resulting encrypted image will be put into an output directory.

mkdir -p oci/{input,output}
skopeo copy docker-daemon:unencrypted:latest dir:./oci/input
docker run -v "${PWD}/oci:/oci" coco-keyprovider /encrypt.sh -k "$KEY_B64" -i "$KEY_ID" -s dir:/oci/input -d dir:/oci/output

We can inspect layer annotations to confirm the expected encryption was applied:

skopeo inspect dir:./oci/output | jq '.LayersData[0].Annotations["org.opencontainers.image.enc.keys.provider.attestation-agent"] | @base64d | fromjson'
{
  "kid": "kbs:///default/image_key/nginx",
  "wrapped_data": "lGaLf2Ge5bwYXHO2g2riJRXyr5a2zrhiXLQnOzZ1LKEQ4ePyE8bWi1GswfBNFkZdd2Abvbvn17XzpOoQETmYPqde0oaYAqVTMcnzTlgdYYzpWZcb3X0ymf9bS0gmMkqO3dPH+Jf4axXuic+ITOKy7MfSVGTLzay6jH/PnSc5TJ2WuUJY2rRtNaTY65kKF2K9YP6mtYBqcHqvPDlFiVNNeTAGv2w1zwaMlgZaSHV+Z1y+xxbOV5e98bxuo6861rMchjCiE7FY37PHD3a5ISogq90=",
  "iv": "Z8bGQL7r6qxSpd4L",
  "wrap_type": "A256GCM"
}

Finally the resulting encrypted image can be provisioned to an image registry.

ENCRYPTED_IMAGE=some-private.registry.io/coco/nginx:encrypted
skopeo copy dir:./oci/output "docker://${ENCRYPTED_IMAGE}"

Provision image key

Prior to launching a Pod the image key needs to be provisioned to the Key Broker’s repository. For a KBS deployment on Kubernetes using the local filesystem as repository storage it would work like this:

kubectl exec deploy/kbs -- mkdir -p "/opt/confidential-containers/kbs/repository/$(dirname "$KEY_PATH")"
cat "$KEY_FILE" | kubectl exec -i deploy/kbs -- tee "/opt/confidential-containers/kbs/repository/${KEY_PATH}" > /dev/null

Launch a Pod

In this example we default to the Cloud API Adaptor runtime, adjust this depending on the CoCo installation.

kubectl get runtimeclass -o jsonpath='{.items[].handler}'
kata-remote
CC_RUNTIMECLASS=kata-remote

We create a simple deployment using our encrypted image. As the image is being pulled and the CoCo components in the TEE encounter the layer annotations that we saw above, the image key will be retrieved from the Key Broker using the annotated Key ID and the layers will be decrypted transparently and the container should come up.

cat <<EOF> nginx-encrypted.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: nginx
  name: nginx-encrypted
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
      annotations:
        io.containerd.cri.runtime-handler: ${CC_RUNTIMECLASS}
    spec:
      runtimeClassName: ${CC_RUNTIMECLASS}
      containers:
      - image: ${ENCRYPTED_IMAGE}
        name: nginx
EOF
kubectl apply -f nginx-encrypted.yaml

We can confirm that the image key has been retrieved from KBS.

kubectl logs -f deploy/kbs | grep "$KEY_PATH"
[2024-01-23T10:24:52Z INFO  actix_web::middleware::logger] 10.244.0.1 "GET /kbs/v0/resource/default/image_key/nginx HTTP/1.1" 200 530 "-" "attestation-agent-kbs-client/0.1.0" 0.000670