Transparent data encryption (TDE) in Azure SQL with customer-managed key (CMK) enables Bring Your Own Key (BYOK) scenario for data protection at rest. With customer-managed TDE, the customer is responsible for and in a full control of a key lifecycle management (key creation, upload, rotation, deletion), key usage permissions, and auditing of operations on keys.

Geo-replication and Failover Groups in Azure SQL Database creates readable secondary replicas across different regions to support disaster recovery and high availability. There are several considerations when configuring geo-replication for Azure SQL logical servers that use Transparent Data Encryption (TDE) with Customer-Managed Keys (CMK). This blog post provides detailed information about setting up geo-replication for TDE-enabled databases.

Understanding the difference between a TDE protector and a server/database Key

To understand the geo-replication considerations, I first need to explain the roles of the TDE protector and a server key.

The TDE protector is the key responsible for encrypting the Database Encryption Key (DEK), which in turn encrypts the actual database files and transaction logs. It sits at the top of the encryption hierarchy. 

A server or database key is a broader concept that refers to any key registered at the server or database level in Azure SQL. One of the registered server or database keys is designated as the TDE protector. Multiple keys can be registered, but only one acts as the active protector at any given time.

Geo-replication considerations

Azure Key Vault considerations

In active geo-replication and failover group scenarios, the primary and secondary SQL logical servers can be linked to an Azure Key Vault in any region — they do not need to be located in the same region. Connecting both SQL logical servers to the same key vault reduces the potential for discrepancies in key material that can occur when using separate key vaults. Azure Key Vault incorporates multiple layers of redundancy to ensure the continued availability of keys and key vaults in the event of service or regional failures.

The following diagram represents a configuration for paired region (primary and secondary) for an Azure Key Vault cross-failover with Azure SQL setup for geo-replication using a failover group.

Azure SQL considerations

The primary consideration is to ensure that the server or database keys are present on both the primary and secondary SQL logical servers or databases, and that appropriate permissions have been granted for these keys within Azure Key Vault. The TDE protector used on the primary does not need to be identical to the one used on the secondary. It is sufficient to have the same key material available on both the primary and secondary systems. You can add keys to a SQL logical server with the Azure Portal, PowerShell, Azure CLI or REST API.

Assign user-assigned managed identities (UMI) to primary and secondary SQL servers for flexibility across regions and failover scenarios. Grant Get, WrapKey, UnwrapKey permissions to these identities on the key vault. For Azure Key Vaults using Azure RBAC for access configuration, the Key Vault Crypto Service Encryption User role is needed by the server identity to be able to use the key for encryption and decryption operations.

Different encryption key combinations

Based on interactions with customers and analysis of livesite incidents involving geo-replication with TDE CMK, we noticed that many clients do not configure the primary and secondary servers with the same TDE protector, due to differing compliance requirements. In this chapter, I will explain how you can set up a failover group with 2 different TDE protectors.

This scenario will use TDECMK key as the TDE protector on the primary server (tdesql)

and TDECMK2 key as the TDE protector on the secondary server (tdedr). As a sidenote, you can also enable the Auto-rotate key to allow end-to-end, zero-touch rotation of the TDE protector 

Both logical SQL Servers have access to the Azure Key Vault keys.

Next step is to create the failover group and replicate 1 database called ContosoHR.

The setup of the failover group failed because the CMK key from the primary server was (intentionally) not added to the secondary server.

Adding a server key can easily be done with the Add-AzSqlServerKeyVaultKey command. For example:

Add-AzSqlServerKeyVaultKey -KeyId 'https://xxxxx.vault.azure.net/keys/TDECMK/ 01234567890123456789012345678901' -ServerName 'tdedr' -ResourceGroupName 'TDEDemo'

After the failover group setup, the sample database on the secondary server becomes available and uses the primary's CMK (TDECMK), regardless of the secondary's CMK configuration. You can verify this by running the following query both on primary and secondary server.

SELECT DB_NAME(db_id()) AS database_name, dek.encryption_state, dek.encryption_state_desc, -- SQL 2019+ / Azure SQL dek.key_algorithm, dek.key_length, dek.encryptor_type, -- CERTIFICATE (service-managed) or ASYMMETRIC KEY (BYOK/CMK) dek.encryptor_thumbprint FROM sys.dm_database_encryption_keys AS dek WHERE database_id <> 2 ;

The encryptor_type column shows ASYMMETRIC KEY, which means the database uses a CMK for encryption. The encryptor_thumbprint should match on both the primary and secondary servers, indicating that the secondary database is encrypted using the CMK from the primary server.  

The TDE protector on the secondary server (TDECMK2) becomes active only in the event of a failover, when the primary and secondary server roles are reversed. As illustrated in the image below, the roles of my primary and secondary servers have been reversed.

If the above query is executed again following a failover, the encryptor_thumbprint value will be different, which indicates that the database is now encrypted using the TDE protector (TDECMK2) from the secondary server.

As previously mentioned, please ensure that the server or database keys are available on both the primary and secondary SQL logical servers or databases. Additionally, verify that the appropriate permissions for these keys have been granted within Azure Key Vault.

This scenario is comparable to other configurations, such as:

• The primary server uses SMK while the secondary server uses CMK.
• The primary server uses CMK while the secondary server uses SMK.

Conclusion

Understanding the distinction between the TDE protector and server keys is essential for geo-replication with Azure SQL. The TDE protector encrypts the database encryption key, while server keys refer to any key registered at the server or database level in Azure SQL. For successful geo-replication setup and failover, all necessary keys must be created and available on both primary and secondary servers. It is possible and, in certain cases, required to configure different TDE protectors on replicas, as long as the key material is available on each server.