Encrypting Confidential Data at Rest

All of the APIs in Kubernetes that let you write persistent API resource data support at-rest encryption. For example, you can enable at-rest encryption for Secrets. This at-rest encryption is additional to any system-level encryption for the etcd cluster or for the filesystem(s) on hosts where you are running the kube-apiserver.

This page shows how to enable and configure encryption of API data at rest.

Before you begin

• You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

  • Killercoda
  • Play with Kubernetes

• This task assumes that you are running the Kubernetes API server as a static pod on each control plane node.

• Your cluster's control plane must use etcd v3.x (major version 3, any minor version).

• To encrypt a custom resource, your cluster must be running Kubernetes v1.26 or newer.

• To use a wildcard to match resources, your cluster must be running Kubernetes v1.27 or newer.

Configuration and determining whether encryption at rest is already enabled

The kube-apiserver process accepts an argument --encryption-provider-config that controls how API data is encrypted in etcd. The configuration is provided as an API named EncryptionConfiguration. An example configuration is provided below.

Understanding the encryption at rest configuration

---
#
# CAUTION: this is an example configuration.
#          Do not use this for your own cluster!
#
apiVersion: apiserver.config.k8s.io/v1
kind: EncryptionConfiguration
resources:
  - resources:
      - secrets
      - configmaps
      - pandas.awesome.bears.example # a custom resource API
    providers:
      # This configuration does not provide data confidentiality. The first
      # configured provider is specifying the "identity" mechanism, which
      # stores resources as plain text.
      #
      - identity: {} # plain text, in other words NO encryption
      - aesgcm:
          keys:
            - name: key1
              secret: c2VjcmV0IGlzIHNlY3VyZQ==
            - name: key2
              secret: dGhpcyBpcyBwYXNzd29yZA==
      - aescbc:
          keys:
            - name: key1
              secret: c2VjcmV0IGlzIHNlY3VyZQ==
            - name: key2
              secret: dGhpcyBpcyBwYXNzd29yZA==
      - secretbox:
          keys:
            - name: key1
              secret: YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXoxMjM0NTY=
  - resources:
      - events
    providers:
      - identity: {} # do not encrypt Events even though *.* is specified below
  - resources:
      - '*.apps' # wildcard match requires Kubernetes 1.27 or later
    providers:
      - aescbc:
          keys:
          - name: key2
            secret: c2VjcmV0IGlzIHNlY3VyZSwgb3IgaXMgaXQ/Cg==
  - resources:
      - '*.*' # wildcard match requires Kubernetes 1.27 or later
    providers:
      - aescbc:
          keys:
          - name: key3
            secret: c2VjcmV0IGlzIHNlY3VyZSwgSSB0aGluaw==

Each resources array item is a separate config and contains a complete configuration. The resources.resources field is an array of Kubernetes resource names (resource or resource.group) that should be encrypted like Secrets, ConfigMaps, or other resources.

If custom resources are added to EncryptionConfiguration and the cluster version is 1.26 or newer, any newly created custom resources mentioned in the EncryptionConfiguration will be encrypted. Any custom resources that existed in etcd prior to that version and configuration will be unencrypted until they are next written to storage. This is the same behavior as built-in resources. See the Ensure all secrets are encrypted section.

The providers array is an ordered list of the possible encryption providers to use for the APIs that you listed. Each provider supports multiple keys - the keys are tried in order for decryption, and if the provider is the first provider, the first key is used for encryption.

Only one provider type may be specified per entry (identity or aescbc may be provided, but not both in the same item). The first provider in the list is used to encrypt resources written into the storage. When reading resources from storage, each provider that matches the stored data attempts in order to decrypt the data. If no provider can read the stored data due to a mismatch in format or secret key, an error is returned which prevents clients from accessing that resource.

EncryptionConfiguration supports the use of wildcards to specify the resources that should be encrypted. Use '*.<group>' to encrypt all resources within a group (for eg '*.apps' in above example) or '*.*' to encrypt all resources. '*.' can be used to encrypt all resource in the core group. '*.*' will encrypt all resources, even custom resources that are added after API server start.

Opting out of encryption for specific resources while wildcard is enabled can be achieved by adding a new resources array item with the resource name, followed by the providers array item with the identity provider. For example, if '*.*' is enabled and you want to opt-out encryption for the events resource, add a new item to the resources array with events as the resource name, followed by the providers array item with identity. The new item should look like this:

- resources:
    - events
  providers:
    - identity: {}

Ensure that the new item is listed before the wildcard '*.*' item in the resources array to give it precedence.

For more detailed information about the EncryptionConfiguration struct, please refer to the encryption configuration API.

Available providers

Before you configure encryption-at-rest for data in your cluster's Kubernetes API, you need to select which provider(s) you will use.

The following table describes each available provider.

The identity provider is the default if you do not specify otherwise. The identity provider does not encrypt stored data and provides no additional confidentiality protection.

Key storage

Local key storage

Encrypting secret data with a locally managed key protects against an etcd compromise, but it fails to protect against a host compromise. Since the encryption keys are stored on the host in the EncryptionConfiguration YAML file, a skilled attacker can access that file and extract the encryption keys.

Managed (KMS) key storage

The KMS provider uses envelope encryption: Kubernetes encrypts resources using a data key, and then encrypts that data key using the managed encryption service. Kubernetes generates a unique data key for each resource. The API server stores an encrypted version of the data key in etcd alongside the ciphertext; when reading the resource, the API server calls the managed encryption service and provides both the ciphertext and the (encrypted) data key. Within the managed encryption service, the provider use a key encryption key to decipher the data key, deciphers the data key, and finally recovers the plain text. Communication between the control plane and the KMS requires in-transit protection, such as TLS.

Using envelope encryption creates dependence on the key encryption key, which is not stored in Kubernetes. In the KMS case, an attacker who intends to get unauthorised access to the plaintext values would need to compromise etcd and the third-party KMS provider.

Write an encryption configuration file

Create a new encryption configuration file. The contents should be similar to:

---
apiVersion: apiserver.config.k8s.io/v1
kind: EncryptionConfiguration
resources:
  - resources:
      - secrets
      - configmaps
      - pandas.awesome.bears.example
    providers:
      - aescbc:
          keys:
            - name: key1
              # See the following text for more details about the secret value
              secret: <BASE 64 ENCODED SECRET>
      - identity: {} # this fallback allows reading unencrypted secrets;
                     # for example, during initial migration

To create a new Secret, perform the following steps:

1. Generate a 32-byte random key and base64 encode it. If you're on Linux or macOS, run the following command:

   head -c 32 /dev/urandom | base64
   

2. Place that value in the secret field of the EncryptionConfiguration struct.

3. Set the --encryption-provider-config flag on the kube-apiserver to point to the location of the config file.

   You will need to mount the new encryption config file to the kube-apiserver static pod. Here is an example on how to do that:

   1. Save the new encryption config file to /etc/kubernetes/enc/enc.yaml on the control-plane node.
   2. Edit the manifest for the kube-apiserver static pod: /etc/kubernetes/manifests/kube-apiserver.yaml similarly to this:

   ---
   #
   # This is a fragment of a manifest for a static Pod.
   # Check whether this is correct for your cluster and for your API server.
   #
   apiVersion: v1
   kind: Pod
   metadata:
     annotations:
       kubeadm.kubernetes.io/kube-apiserver.advertise-address.endpoint: 10.20.30.40:443
     creationTimestamp: null
     labels:
       app.kubernetes.io/component: kube-apiserver
       tier: control-plane
     name: kube-apiserver
     namespace: kube-system
   spec:
     containers:
     - command:
       - kube-apiserver
       ...
       - --encryption-provider-config=/etc/kubernetes/enc/enc.yaml  # add this line
       volumeMounts:
       ...
       - name: enc                           # add this line
         mountPath: /etc/kubernetes/enc      # add this line
         readOnly: true                      # add this line
       ...
     volumes:
     ...
     - name: enc                             # add this line
       hostPath:                             # add this line
         path: /etc/kubernetes/enc           # add this line
         type: DirectoryOrCreate             # add this line
     ...
   

4. Restart your API server.

Reconfigure other control plane hosts

If you have multiple API servers in your cluster, you should deploy the changes in turn to each API server.

Make sure that you use the same encryption configuration on each control plane host.

Verify that newly written data is encrypted

Data is encrypted when written to etcd. After restarting your kube-apiserver, any newly created or updated Secret (or other resource kinds configured in EncryptionConfiguration) should be encrypted when stored.

To check this, you can use the etcdctl command line program to retrieve the contents of your secret data.

This example shows how to check this for encrypting the Secret API.

1. Create a new Secret called secret1 in the default namespace:

   kubectl create secret generic secret1 -n default --from-literal=mykey=mydata
   

2. Using the etcdctl command line tool, read that Secret out of etcd:

   ETCDCTL_API=3 etcdctl get /registry/secrets/default/secret1 [...] | hexdump -C
   

   where [...] must be the additional arguments for connecting to the etcd server.

   For example:

   ETCDCTL_API=3 etcdctl \
      --cacert=/etc/kubernetes/pki/etcd/ca.crt   \
      --cert=/etc/kubernetes/pki/etcd/server.crt \
      --key=/etc/kubernetes/pki/etcd/server.key  \
      get /registry/secrets/default/secret1 | hexdump -C
   

   The output is similar to this (abbreviated):

   00000000  2f 72 65 67 69 73 74 72  79 2f 73 65 63 72 65 74  |/registry/secret|
   00000010  73 2f 64 65 66 61 75 6c  74 2f 73 65 63 72 65 74  |s/default/secret|
   00000020  31 0a 6b 38 73 3a 65 6e  63 3a 61 65 73 63 62 63  |1.k8s:enc:aescbc|
   00000030  3a 76 31 3a 6b 65 79 31  3a c7 6c e7 d3 09 bc 06  |:v1:key1:.l.....|
   00000040  25 51 91 e4 e0 6c e5 b1  4d 7a 8b 3d b9 c2 7c 6e  |%Q...l..Mz.=..|n|
   00000050  b4 79 df 05 28 ae 0d 8e  5f 35 13 2c c0 18 99 3e  |.y..(..._5.,...>|
   [...]
   00000110  23 3a 0d fc 28 ca 48 2d  6b 2d 46 cc 72 0b 70 4c  |#:..(.H-k-F.r.pL|
   00000120  a5 fc 35 43 12 4e 60 ef  bf 6f fe cf df 0b ad 1f  |..5C.N`..o......|
   00000130  82 c4 88 53 02 da 3e 66  ff 0a                    |...S..>f..|
   0000013a
   

3. Verify the stored Secret is prefixed with k8s:enc:aescbc:v1: which indicates the aescbc provider has encrypted the resulting data. Confirm that the key name shown in etcd matches the key name specified in the EncryptionConfiguration mentioned above. In this example, you can see that the encryption key named key1 is used in etcd and in EncryptionConfiguration.

4. Verify the Secret is correctly decrypted when retrieved via the API:

   kubectl get secret secret1 -n default -o yaml
   

   The output should contain mykey: bXlkYXRh, with contents of mydata encoded using base64; read decoding a Secret to learn how to completely decode the Secret.

Ensure all relevant data are encrypted

It's often not enough to make sure that new objects get encrypted: you also want that encryption to apply to the objects that are already stored.

For this example, you have configured your cluster so that Secrets are encrypted on write. Performing a replace operation for each Secret will encrypt that content at rest, where the objects are unchanged.

You can make this change across all Secrets in your cluster:

# Run this as an administrator that can read and write all Secrets
kubectl get secrets --all-namespaces -o json | kubectl replace -f -

The command above reads all Secrets and then updates them with the same data, in order to apply server side encryption.

Rotating a decryption key

Changing a Secret without incurring downtime requires a multi-step operation, especially in the presence of a highly-available deployment where multiple kube-apiserver processes are running.

1. Generate a new key and add it as the second key entry for the current provider on all servers
2. Restart all kube-apiserver processes to ensure each server can decrypt using the new key
3. Make the new key the first entry in the keys array so that it is used for encryption in the config
4. Restart all kube-apiserver processes to ensure each server now encrypts using the new key
5. Run kubectl get secrets --all-namespaces -o json | kubectl replace -f - to encrypt all existing Secrets with the new key
6. Remove the old decryption key from the config after you have backed up etcd with the new key in use and updated all Secrets

When running a single kube-apiserver instance, step 2 may be skipped.

Configure automatic reloading

You can configure automatic reloading of encryption provider configuration. That setting determines whether the API server should load the file you specify for --encryption-provider-config only once at startup, or automatically whenever you change that file. Enabling this option allows you to change the keys for encryption at rest without restarting the API server.

To allow automatic reloading, configure the API server to run with: --encryption-provider-config-automatic-reload=true

What's next

• Read about decrypting data that are already stored at rest
• Learn more about the EncryptionConfiguration configuration API (v1).