doc: proposal for providing PV key rotation

The design and implementation details for rotating
the encryption keys for volumes.

Signed-off-by: black-dragon74 <niryadav@redhat.com>

docs/design/proposals/pv-key-rotation.md

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+# Encryption Key Rotation
+
+## Proposal
+
+Subject of this proposal is to add support for rotation of encryption keys (KEKs) for encrypted volumes in Ceph-CSI.
+
+## Document Terminology
+
+- Encryption Key: The passphrase that is used to encrypt and open the device.
+- LUKS: The specification used by dm-crypt to process encrypted volumes on linux.
+
+## Proposed Solution
+
+The proposed solution in this document, is to address the periodic rotation of encryption keys for encrypted volumes.
+
+This document outlines the rotation steps for PVCs backed by RBD and will be updated with other volume types as they are supported.
+
+### Implementation Summary
+
+This feature builds upon the foundation laid by encrypted pvcs.
+
+An existing storage class can be annotated with `keyrotation.csiaddons.openshift.io/schedule` to enable the key rotation. The value of this annotation can be schedule in cron format or one of the macros supported by K8s CronJob spec.
+
+The following new methods are added to `cryptsetup.go` for handling the key rotation.
+
+- `LuksAddKey`: Adds a new key to specified LUKS slot
+- `LuksRemoveKey`: Removes the specified key from its slot using `luksKillSlot`
+- `LuksVerifyKey`: Verifies that the given key exists in the given slot using `luksChangeKey`.
+
+### Implementation Details
+
+The encryption key rotation request will contain with it the volume ID, credentials and secrets.
+
+These values are then used to call `GenVolFromVolID` to get the rbdVolume structure.
+
+The `VolumeEncryption` struct is modified to make `generateNewEncryptionPassphrase` a public member function.
+
+A metadata is set on the RBD image to indicate that the image is being processed for keyrotation. Presence of this metadata will prevent the same image being processed again.
+
+The following steps are followed to process the device for key rotation:
+
+- Create a `rbdvolume` object using volume ID, this is done by `GenVolFromVolID`.
+- Fetch the current key from the KMS, it is needed for subsequent LUKS operations.
+- Get the device path for the volume by calling `waitForPath` as all LUKS operations require the device path.
+- Add the fetched key to LUKS slot 1, this will serve as a backup of the key.
+- Generate a new key and store it locally. It will be updated in the KMS at later steps.
+- Remove the exsitng key from slot 0 upon verifying that the key in KMS == the key in slot 0.
+- Add new key to slot 0 and then call `LuksVerifyKey` to verify that the slot was successfully updated.
+- Update the new key in the KMS.
+- Fetch the key again and verify that the key in KMS == the new key we generated.
+- We can now remove the backup key from slot 1.
+
+These order of the above steps guarantees that we always have one key that can unlock the encrypted volume.
+
+The set metadata is removed once the key rotation is complete.