Document avoiding plain text retrieval

When you have set up your cluster for encryption at rest, you can take
this defence in depth measure to make sure that anything held without
encryption causes a retrieval error (which is then more likely to flag
that there is a problem).

Author: Tim Bannister

content/en/docs/tasks/administer-cluster/encrypt-data.md

@@ -562,6 +562,43 @@ For larger clusters, you may wish to subdivide the Secrets by namespace,
 or script an update.
 {{< /note >}}
 
+## Prevent plain text retrieval {#cleanup-all-secrets-encrypted}
+
+If you want to make sure that the only access to a particular API kind is done using
+encryption, you can remove the API server's ability to read that API's backing data
+as plaintext.
+
+{{< warning >}}
+Making this change prevents the API server from retrieving resources that are marked
+as encrypted as rest, but are actually stored in the clear.
+
+When you have configured encryption at rest for an API (for example: the API kind
+`Secret`, representing `secrets` resources in the core API group), you **must** ensure
+that all those resources in this cluster really are encrypted at rest. Check this before
+you carry on with the next steps.
+{{< /warning >}}
+
+Once all Secrets in your cluster are encrypted, you can remove the `identity`
+part of the encryption configuration. For example:
+
+{{< highlight yaml "linenos=false,hl_lines=12" >}}
+---
+apiVersion: apiserver.config.k8s.io/v1
+kind: EncryptionConfiguration
+resources:
+  - resources:
+      - secrets
+    providers:
+      - aescbc:
+          keys:
+            - name: key1
+              secret: <BASE 64 ENCODED SECRET>
+      - identity: {} # REMOVE THIS LINE
+{{< /highlight >}}
+
+…and then restart each API server in turn. This change prevents the API server
+from accessing a plain-text Secret, even by accident.
+
 ## Rotating a decryption key
 
 Changing a Secret without incurring downtime requires a multi-step operation, especially in