Transparent Data Encryption (TDE) is a feature in SQL Server that allows you to encrypt your database at rest. It provides an additional layer of security by encrypting the data files, log files, and backups. In this article, we will explore how TDE can be used in conjunction with SQL Server Availability Groups.

Basic Operations with Certificates

Before we dive into TDE on Availability Groups, let’s first understand some basic operations with certificates that are crucial for managing TDE. One important operation is backing up a certificate. To back up a certificate, you can use the following script:

USE [master];
GO
BACKUP CERTIFICATE MyNewTDECert
TO FILE = '\\shared_folder\my_secrets\MyNewTDECert.cer'
WITH PRIVATE KEY (FILE = '\\shared_folder\my_secrets\MyNewTDECert.key',
ENCRYPTION BY PASSWORD = 'Super$ecretPassW0rd111');
GO

This script takes a backup of the certificate ‘MyNewTDECert’ including the private key in two files on a shared location. It is important to keep these files in a secure location and the password in an encrypted form.

Another important task is certificate restoration. You can restore a certificate using the following script:

USE [master];
GO
CREATE CERTIFICATE MyNewTDECert
FROM FILE = '\\shared_folder\my_secrets\MyNewTDECert.cer'
WITH PRIVATE KEY (FILE = '\\shared_folder\my_secrets\MyNewTDECert.key',
DECRYPTION BY PASSWORD = 'Super$ecretPassW0rd111');
GO

When restoring an expired certificate, you may see a warning message, but TDE will still work fine. It is recommended to renew all expired certificates.

Replacing an Expired TDE Certificate

To replace an expired TDE certificate, you need to create a new one. You can use the script mentioned earlier to create a new certificate with a custom expiration date. After creating the new certificate, you can use the following script to replace the encryption certificate for your database:

USE [MySecretDatabase];
GO
ALTER DATABASE ENCRYPTION KEY
ENCRYPTION BY SERVER CERTIFICATE MyNewTDECert;
GO

Make sure to take backups of all certificates used for TDE and check that the TDE database is using the new certificate.

Replacing an Expired TDE Certificate on a Database in an Availability Group

Before making any changes, ensure that the Availability Group is healthy and the database is synchronized across the replicas. To replace the TDE certificate in an Availability Group, follow these steps:

1. Create the new certificate on the primary node.
2. Create a backup of the certificate with the private key.
3. Distribute the certificate with the private key among all the AG nodes.
4. Alter the database encryption key on the primary replica.
5. Drop the old certificate from all the replicas.

After renewing the TDE certificate, ensure that the Availability Group is healthy and the TDE is in a synchronized state.

Automatic Database Seeding with a TDE Database

If you want to use TDE databases with automatic database seeding, you must distribute encryption certificates among all the participating replicas before adding a database into an AG. Follow these steps:

1. Enable TDE on the primary replica.
2. Backup the TDE certificate from the primary replica.
3. Distribute the TDE certificate among all participating replicas.
4. Optionally, verify that the correct certificate is distributed among all replicas.
5. Follow the instructions for adding a database with automatic seeding using T-SQL scripts.

Remember that in the case of a TDE database, you cannot use the GUI option for adding a database to an AG.

Conclusion

Combining SQL Server features like TDE, Availability Groups, and Automatic Database Seeding can be challenging due to various limitations. However, as we have seen in this article, it is possible to use TDE on Availability Groups with some additional steps and careful management of certificates. By understanding the basic operations with certificates and following the recommended steps, you can ensure the security and availability of your TDE-enabled databases.