Using a KMS provider for data encryption

Using a KMS provider for data encryption

This page shows how to configure a Key Management Service (KMS) provider and plugin to enable secret data encryption. In Kubernetes 1.28 there are two versions of KMS at-rest encryption. You should use KMS v2 if feasible because KMS v1 is deprecated (since Kubernetes v1.28). However, you should also read and observe the Caution notices in this page that highlight specific cases when you must not use KMS v2. KMS v2 offers significantly better performance characteristics than KMS v1.

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:

The version of Kubernetes that you need depends on which KMS API version you have selected. Kubernetes recommends using KMS v2.

If you selected KMS API v2, you should use Kubernetes v1.28 (if you are running a different version of Kubernetes that also supports the v2 KMS API, switch to the documentation for that version of Kubernetes).
If you selected KMS API v1 to support clusters prior to version v1.27 or if you have a legacy KMS plugin that only supports KMS v1, any supported Kubernetes version will work. This API is deprecated as of Kubernetes v1.28. Kubernetes does not recommend the use of this API.

To check the version, enter kubectl version.

KMS v1

FEATURE STATE: Kubernetes v1.28 [deprecated]

Kubernetes version 1.10.0 or later is required
Your cluster must use etcd v3 or later

KMS v2

FEATURE STATE: Kubernetes v1.27 [beta]

For version 1.25 and 1.26, enabling the feature via kube-apiserver feature gate is required. Set --feature-gates=KMSv2=true to configure a KMS v2 provider. For environments where all API servers are running version 1.28 or later, and you do not require the ability to downgrade to Kubernetes v1.27, you can enable the KMSv2KDF feature gate (a beta feature) for more robust data encryption key generation. The Kubernetes project recommends enabling KMS v2 KDF if those preconditions are met.
Your cluster must use etcd v3 or later

Caution:

The KMS v2 API and implementation changed in incompatible ways in-between the alpha release in v1.25 and the beta release in v1.27. Attempting to upgrade from old versions with the alpha feature enabled will result in data loss.

Running mixed API server versions with some servers at v1.27, and others at v1.28 with the KMSv2KDF feature gate enabled is not supported - and is likely to result in data loss.

The KMS encryption provider uses an envelope encryption scheme to encrypt data in etcd. The data is encrypted using a data encryption key (DEK). The DEKs are encrypted with a key encryption key (KEK) that is stored and managed in a remote KMS.

With KMS v1, a new DEK is generated for each encryption.

With KMS v2, there are two ways for the API server to generate a DEK. Kubernetes defaults to generating a new DEK at API server startup, which is then reused for resource encryption. However, if you use KMS v2 and enable the KMSv2KDF feature gate, then Kubernetes instead generates a new DEK per encryption: the API server uses a key derivation function to generate single use data encryption keys from a secret seed combined with some random data. Whichever approach you configure, the DEK or seed is also rotated whenever the KEK is rotated (see Understanding key_id and Key Rotation section below for more details).

The KMS provider uses gRPC to communicate with a specific KMS plugin over a UNIX domain socket. The KMS plugin, which is implemented as a gRPC server and deployed on the same host(s) as the Kubernetes control plane, is responsible for all communication with the remote KMS.

Caution:

If you are running virtual machine (VM) based nodes that leverage VM state store with this feature, using KMS v2 is insecure and an information security risk unless you also explicitly enable the KMSv2KDF feature gate.

With KMS v2, the API server uses AES-GCM with a 12 byte nonce (8 byte atomic counter and 4 bytes random data) for encryption. The following issues could occur if the VM is saved and restored:

The counter value may be lost or corrupted if the VM is saved in an inconsistent state or restored improperly. This can lead to a situation where the same counter value is used twice, resulting in the same nonce being used for two different messages.
If the VM is restored to a previous state, the counter value may be set back to its previous value, resulting in the same nonce being used again.

Although both of these cases are partially mitigated by the 4 byte random nonce, this can compromise the security of the encryption.

If you have enabled the KMSv2KDF feature gate and are using KMS v2 (not KMS v1), the API server generates single use data encryption keys from a secret seed. This eliminates the need for a counter based nonce while avoiding nonce collision concerns. It also removes any specific concerns with using KMS v2 and VM state store.

Configuring the KMS provider

To configure a KMS provider on the API server, include a provider of type kms in the providers array in the encryption configuration file and set the following properties:

KMS v1

apiVersion: API Version for KMS provider. Leave this value empty or set it to v1.
name: Display name of the KMS plugin. Cannot be changed once set.
endpoint: Listen address of the gRPC server (KMS plugin). The endpoint is a UNIX domain socket.
cachesize: Number of data encryption keys (DEKs) to be cached in the clear. When cached, DEKs can be used without another call to the KMS; whereas DEKs that are not cached require a call to the KMS to unwrap.
timeout: How long should kube-apiserver wait for kms-plugin to respond before returning an error (default is 3 seconds).

KMS v2

apiVersion: API Version for KMS provider. Set this to v2.
name: Display name of the KMS plugin. Cannot be changed once set.
endpoint: Listen address of the gRPC server (KMS plugin). The endpoint is a UNIX domain socket.
timeout: How long should kube-apiserver wait for kms-plugin to respond before returning an error (default is 3 seconds).

KMS v2 does not support the cachesize property. All data encryption keys (DEKs) will be cached in the clear once the server has unwrapped them via a call to the KMS. Once cached, DEKs can be used to perform decryption indefinitely without making a call to the KMS.

See Understanding the encryption at rest configuration.

Implementing a KMS plugin

To implement a KMS plugin, you can develop a new plugin gRPC server or enable a KMS plugin already provided by your cloud provider. You then integrate the plugin with the remote KMS and deploy it on the Kubernetes control plane.

Enabling the KMS supported by your cloud provider

Refer to your cloud provider for instructions on enabling the cloud provider-specific KMS plugin.

Developing a KMS plugin gRPC server

You can develop a KMS plugin gRPC server using a stub file available for Go. For other languages, you use a proto file to create a stub file that you can use to develop the gRPC server code.

KMS v1

Using Go: Use the functions and data structures in the stub file: api.pb.go to develop the gRPC server code
Using languages other than Go: Use the protoc compiler with the proto file: api.proto to generate a stub file for the specific language

KMS v2

Using Go: A high level library is provided to make the process easier. Low level implementations can use the functions and data structures in the stub file: api.pb.go to develop the gRPC server code
Using languages other than Go: Use the protoc compiler with the proto file: api.proto to generate a stub file for the specific language

Then use the functions and data structures in the stub file to develop the server code.

Notes

KMS v1

kms plugin version: v1beta1

In response to procedure call Version, a compatible KMS plugin should return v1beta1 as VersionResponse.version.
message version: v1beta1

All messages from KMS provider have the version field set to v1beta1.
protocol: UNIX domain socket (unix)

The plugin is implemented as a gRPC server that listens at UNIX domain socket. The plugin deployment should create a file on the file system to run the gRPC unix domain socket connection. The API server (gRPC client) is configured with the KMS provider (gRPC server) unix domain socket endpoint in order to communicate with it. An abstract Linux socket may be used by starting the endpoint with /@, i.e. unix:///@foo. Care must be taken when using this type of socket as they do not have concept of ACL (unlike traditional file based sockets). However, they are subject to Linux networking namespace, so will only be accessible to containers within the same pod unless host networking is used.

KMS v2

KMS plugin version: v2beta1

In response to procedure call Status, a compatible KMS plugin should return v2beta1 as StatusResponse.version, “ok” as StatusResponse.healthz and a key_id (remote KMS KEK ID) as StatusResponse.key_id.

The API server polls the Status procedure call approximately every minute when everything is healthy, and every 10 seconds when the plugin is not healthy. Plugins must take care to optimize this call as it will be under constant load.
Encryption

The EncryptRequest procedure call provides the plaintext and a UID for logging purposes. The response must include the ciphertext, the key_id for the KEK used, and, optionally, any metadata that the KMS plugin needs to aid in future DecryptRequest calls (via the annotations field). The plugin must guarantee that any distinct plaintext results in a distinct response (ciphertext, key_id, annotations).

If the plugin returns a non-empty annotations map, all map keys must be fully qualified domain names such as example.com. An example use case of annotation is {"kms.example.io/remote-kms-auditid":"<audit ID used by the remote KMS>"}

The API server does not perform the EncryptRequest procedure call at a high rate. Plugin implementations should still aim to keep each request’s latency at under 100 milliseconds.
Decryption

The DecryptRequest procedure call provides the (ciphertext, key_id, annotations) from EncryptRequest and a UID for logging purposes. As expected, it is the inverse of the EncryptRequest call. Plugins must verify that the key_id is one that they understand - they must not attempt to decrypt data unless they are sure that it was encrypted by them at an earlier time.

The API server may perform thousands of DecryptRequest procedure calls on startup to fill its watch cache. Thus plugin implementations must perform these calls as quickly as possible, and should aim to keep each request’s latency at under 10 milliseconds.
Understanding key_id and Key Rotation

The key_id is the public, non-secret name of the remote KMS KEK that is currently in use. It may be logged during regular operation of the API server, and thus must not contain any private data. Plugin implementations are encouraged to use a hash to avoid leaking any data. The KMS v2 metrics take care to hash this value before exposing it via the /metrics endpoint.

The API server considers the key_id returned from the Status procedure call to be authoritative. Thus, a change to this value signals to the API server that the remote KEK has changed, and data encrypted with the old KEK should be marked stale when a no-op write is performed (as described below). If an EncryptRequest procedure call returns a key_id that is different from Status, the response is thrown away and the plugin is considered unhealthy. Thus implementations must guarantee that the key_id returned from Status will be the same as the one returned by EncryptRequest. Furthermore, plugins must ensure that the key_id is stable and does not flip-flop between values (i.e. during a remote KEK rotation).

Plugins must not re-use key_ids, even in situations where a previously used remote KEK has been reinstated. For example, if a plugin was using key_id=A, switched to key_id=B, and then went back to key_id=A - instead of reporting key_id=A the plugin should report some derivative value such as key_id=A_001 or use a new value such as key_id=C.

Since the API server polls Status about every minute, key_id rotation is not immediate. Furthermore, the API server will coast on the last valid state for about three minutes. Thus if a user wants to take a passive approach to storage migration (i.e. by waiting), they must schedule a migration to occur at 3 + N + M minutes after the remote KEK has been rotated (N is how long it takes the plugin to observe the key_id change and M is the desired buffer to allow config changes to be processed - a minimum M of five minutes is recommend). Note that no API server restart is required to perform KEK rotation.

Caution: Because you don’t control the number of writes performed with the DEK, the Kubernetes project recommends rotating the KEK at least every 90 days.
protocol: UNIX domain socket (unix)

The plugin is implemented as a gRPC server that listens at UNIX domain socket. The plugin deployment should create a file on the file system to run the gRPC unix domain socket connection. The API server (gRPC client) is configured with the KMS provider (gRPC server) unix domain socket endpoint in order to communicate with it. An abstract Linux socket may be used by starting the endpoint with /@, i.e. unix:///@foo. Care must be taken when using this type of socket as they do not have concept of ACL (unlike traditional file based sockets). However, they are subject to Linux networking namespace, so will only be accessible to containers within the same pod unless host networking is used.

Integrating a KMS plugin with the remote KMS

The KMS plugin can communicate with the remote KMS using any protocol supported by the KMS. All configuration data, including authentication credentials the KMS plugin uses to communicate with the remote KMS, are stored and managed by the KMS plugin independently. The KMS plugin can encode the ciphertext with additional metadata that may be required before sending it to the KMS for decryption (KMS v2 makes this process easier by providing a dedicated annotations field).

Deploying the KMS plugin

Ensure that the KMS plugin runs on the same host(s) as the Kubernetes API server(s).

Encrypting your data with the KMS provider

To encrypt the data:

Create a new EncryptionConfiguration file using the appropriate properties for the kms provider to encrypt resources like Secrets and ConfigMaps. If you want to encrypt an extension API that is defined in a CustomResourceDefinition, your cluster must be running Kubernetes v1.26 or newer.
Set the --encryption-provider-config flag on the kube-apiserver to point to the location of the configuration file.
--encryption-provider-config-automatic-reload boolean argument determines if the file set by --encryption-provider-config should be automatically reloaded if the disk contents change. This enables key rotation without API server restarts.
Restart your API server.

KMS v1

apiVersion: apiserver.config.k8s.io/v1
kind: EncryptionConfiguration
resources:
  - resources:
      - secrets
      - configmaps
      - pandas.awesome.bears.example
    providers:
      - kms:
          name: myKmsPluginFoo
          endpoint: unix:///tmp/socketfile.sock
          cachesize: 100
          timeout: 3s
      - kms:
          name: myKmsPluginBar
          endpoint: unix:///tmp/socketfile.sock
          cachesize: 100
          timeout: 3s

KMS v2

apiVersion: apiserver.config.k8s.io/v1
kind: EncryptionConfiguration
resources:
  - resources:
      - secrets
      - configmaps
      - pandas.awesome.bears.example
    providers:
      - kms:
          apiVersion: v2
          name: myKmsPluginFoo
          endpoint: unix:///tmp/socketfile.sock
          timeout: 3s
      - kms:
          apiVersion: v2
          name: myKmsPluginBar
          endpoint: unix:///tmp/socketfile.sock
          timeout: 3s

Setting --encryption-provider-config-automatic-reload to true collapses all health checks to a single health check endpoint. Individual health checks are only available when KMS v1 providers are in use and the encryption config is not auto-reloaded.

The following table summarizes the health check endpoints for each KMS version:

KMS configurations	Without Automatic Reload	With Automatic Reload
KMS v1 only	Individual Healthchecks	Single Healthcheck
KMS v2 only	Single Healthcheck	Single Healthcheck
Both KMS v1 and v2	Individual Healthchecks	Single Healthcheck
No KMS	None	Single Healthcheck

Single Healthcheck means that the only health check endpoint is /healthz/kms-providers.

Individual Healthchecks means that each KMS plugin has an associated health check endpoint based on its location in the encryption config: /healthz/kms-provider-0, /healthz/kms-provider-1 etc.

These healthcheck endpoint paths are hard coded and generated/controlled by the server. The indices for individual healthchecks corresponds to the order in which the KMS encryption config is processed.

At a high level, restarting an API server when a KMS plugin is unhealthy is unlikely to make the situation better. It can make the situation significantly worse by throwing away the API server’s DEK cache. Thus the general recommendation is to ignore the API server KMS healthz checks for liveness purposes, i.e. /livez?exclude=kms-providers.

Until the steps defined in Ensuring all secrets are encrypted are performed, the providers list should end with the identity: {} provider to allow unencrypted data to be read. Once all resources are encrypted, the identity provider should be removed to prevent the API server from honoring unencrypted data.

For details about the EncryptionConfiguration format, please check the API server encryption API reference.

Verifying that the data is encrypted

When encryption at rest is correctly configured, resources are encrypted on write. After restarting your kube-apiserver, any newly created or updated Secret or other resource types configured in EncryptionConfiguration should be encrypted when stored. To verify, you can use the etcdctl command line program to retrieve the contents of your secret data.

Create a new secret called secret1 in the default namespace:

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

Using the etcdctl command line, read that secret out of etcd:
```
ETCDCTL_API=3 etcdctl get /kubernetes.io/secrets/default/secret1 [...] | hexdump -C
```
where [...] contains the additional arguments for connecting to the etcd server.
Verify the stored secret is prefixed with k8s:enc:kms:v1: for KMS v1 or prefixed with k8s:enc:kms:v2: for KMS v2, which indicates that the kms provider has encrypted the resulting data.
Verify that the secret is correctly decrypted when retrieved via the API:
```
kubectl describe secret secret1 -n default
```
The Secret should contain mykey: mydata

Ensuring all secrets are encrypted

When encryption at rest is correctly configured, resources are encrypted on write. Thus we can perform an in-place no-op update to ensure that data is encrypted.

The following command reads all secrets and then updates them to apply server side encryption. If an error occurs due to a conflicting write, retry the command. For larger clusters, you may wish to subdivide the secrets by namespace or script an update.

kubectl get secrets --all-namespaces -o json | kubectl replace -f -

Switching from a local encryption provider to the KMS provider

To switch from a local encryption provider to the kms provider and re-encrypt all of the secrets:

Add the kms provider as the first entry in the configuration file as shown in the following example.

apiVersion: apiserver.config.k8s.io/v1
kind: EncryptionConfiguration
resources:
  - resources:
      - secrets
    providers:
      - kms:
          apiVersion: v2
          name : myKmsPlugin
          endpoint: unix:///tmp/socketfile.sock
      - aescbc:
          keys:
            - name: key1
              secret: <BASE 64 ENCODED SECRET>

Restart all kube-apiserver processes.
Run the following command to force all secrets to be re-encrypted using the kms provider.
```
kubectl get secrets --all-namespaces -o json | kubectl replace -f -
```

Disabling encryption at rest

To disable encryption at rest:

Place the identity provider as the first entry in the configuration file:

apiVersion: apiserver.config.k8s.io/v1
kind: EncryptionConfiguration
resources:
  - resources:
      - secrets
    providers:
      - identity: {}
      - kms:
          apiVersion: v2
          name : myKmsPlugin
          endpoint: unix:///tmp/socketfile.sock

Restart all kube-apiserver processes.

Run the following command to force all secrets to be decrypted.

kubectl get secrets --all-namespaces -o json | kubectl replace -f -