doc: added docs for Azure KMS

Signed-off-by: Praveen M <m.praveen@ibm.com>

PendingReleaseNotes.md

@@ -4,4 +4,6 @@
 
 ## Features
 
+- kms: added Azure Key Vault as a supported KMS in [PR](https://github.com/ceph/ceph-csi/pull/4455)
+
 ## NOTE

docs/deploy-rbd.md

@@ -448,6 +448,33 @@ the AWS KMS is expected to contain:
 This Secret is expected to be created by the tenant/user in each namespace where
 Ceph-CSI is used to create encrypted rbd volumes.
 
+#### Configuring Azure key vault
+
+Ceph-CSI can be configured to use
+[Azure key vault](https://azure.microsoft.com/en-in/products/key-vault),
+for encrypting RBD volumes.
+
+There are a few settings that need to be included in the [KMS configuration
+file](../examples/kms/vault/kms-config.yaml):
+
+1. `KMS_PROVIDER`: should be set to `azure-kv`.
+1. `AZURE_CERT_SECRET_NAME`: name of the Kubernetes Secret (in the Namespace where
+   Ceph-CSI is deployed) which contains the credentials for communicating with
+   Azure. This defaults to `ceph-csi-azure-credentials`.
+1. `AZURE_VAULT_URL`: URL to access the Azure Key Vault service.
+1. `AZURE_CLIENT_ID`: Client ID of the Azure application object (service principal)
+   created in Azure Active Directory that serves as the username.
+1. `AZURE_TENANT_ID`: Tenant ID of the service principal.
+
+The [Secret with credentials](../examples/kms/vault/azure-credentials.yaml) for
+the Azure KMS is expected to contain:
+
+1. `CLIENT_CERT`: The client certificate used for authentication
+   with Azure Key Vault.
+
+This Secret is expected to be created by the user in the namespace where Ceph-CSI
+is deployed.
+
 #### Configuring KMIP KMS
 
 The Key Management Interoperability Protocol (KMIP) is an extensible

docs/design/proposals/encryption-with-azure-keyvault.md

@@ -0,0 +1,58 @@
+# Encrypted volumes with Azure Key Vault
+
+Azure Key Vault is a cloud service for securely storing and accessing secrets.
+A secret is anything that you want to tightly control access to, such as API
+keys, passwords, certificates, or cryptographic keys.
+
+## Connection to Azure Key Vault
+
+Below values are used to establish the connection to the Key Vault
+service from the CSI driver and to make use of the secrets
+`GetSecret`/`SetSecret`/`DeleteSecret` operations:
+
+```text
+* AZURE_VAULT_URL
+The URL used to access the Azure Key Vault service.
+
+* AZURE_CLIENT_ID
+The Client ID of the Azure application object (also known as the service principal).
+This ID serves as the username.
+
+* AZURE_TENANT_ID
+The Tenant ID associated with the service principal.
+
+* CLIENT_CERT
+The client certificate (which includes the private key and is not password protected)
+used for authentication with Azure Key Vault.
+```
+
+### Values provided in the connection secret
+
+Considering `AZURE_CLIENT_CERTIFICATE` is sensitive information,
+it will be provided as a Kubernetes secret to the Ceph-CSI driver. The Ceph-CSI
+KMS plugin interface for the Azure key vault will read the secret name from the
+kms configMap and fetch the certificate.
+
+### Values provided in the config map
+
+`AZURE_VAULT_URL`, `AZURE_CLIENT_ID`, `AZURE_TENANT_ID` are part of the
+KMS ConfigMap.
+
+### Storage class values or configuration
+
+The Storage class has to be enabled for encryption and `encryptionKMSID` has
+to be provided which is the matching value in the kms config map.
+
+## Volume Encrypt or Decrypt Operation
+
+Ceph-CSI generate's unique passphrase for each volume to be used to
+encrypt/decrypt. The passphrase is securely store in Azure key vault
+using the `SetSecret` operation. At time of decrypt the passphrase is
+retrieved from the key vault using the `GetSecret`operation.
+
+## Volume Delete Operation
+
+When the corresponding volume is deleted, the stored secret in the Azure Key
+Vault will be deleted.
+
+> Note: Ceph-CSI solely deletes the secret without permanent removal (purging).