Transparent Data Encryption (TDE)

Applies to: SQL Server 2008 and later versions

Transparent Data Encryption (TDE) encrypts data files of a SQL Server database at rest. This includes the data and log files. TDE also encrypts backups. TDE encrypts data at the storage level, not at the application level.

How TDE Works

TDE protects data against the threat of unauthorized access to the physical files or their copies. It works by encrypting the entire database, including data and log files, using a symmetric key called the Database Encryption Key (DEK). This DEK is then protected by a Service Master Key (SMK), which is a master key managed by the SQL Server instance. For enhanced security and manageability, the DEK can also be protected by an Asymmetric Key or an X.509 Certificate.

Important: TDE encrypts data at rest, meaning the files on disk. It does not encrypt data in transit (network traffic) or data while it is being processed in memory. For these scenarios, consider other encryption methods like Always Encrypted or SSL/TLS.

Key Components of TDE

Database Encryption Key (DEK): A symmetric key used to encrypt the database's data and log files.
Service Master Key (SMK): A symmetric key managed by the SQL Server instance that encrypts the DEK.
Master Key (Optional): Can be an Asymmetric Key or an X.509 Certificate used to encrypt the DEK, providing an additional layer of security and manageability.

Benefits of TDE

Data Security: Protects sensitive data from unauthorized access to database files.
Compliance: Helps meet regulatory compliance requirements for data protection.
Ease of Use: Transparent to applications; no application code changes are required.
Performance: While there is some overhead, TDE is generally efficient for encrypting data at rest.

Steps to Implement TDE

Implementing TDE involves several steps, primarily focused on creating and managing encryption keys:

Create a Master Key: You can create a master key for the database, typically an asymmetric key or a certificate.

-- Example: Creating a certificate for TDE
CREATE CERTIFICATE MyDatabaseCert
   WITH SUBJECT = 'TDE Certificate for MyDatabase',
   EXPIRY_DATE = '2025-12-31';
GO

-- Example: Creating an asymmetric key to protect the certificate
CREATE ASYMMETRIC KEY MyDatabaseKey
   FROM CERTIFICATE MyDatabaseCert;
GO

-- Example: Creating a master key for the database
CREATE MASTER KEY ENCRYPTION BY ASYMMETRIC KEY MyDatabaseKey;
GO

Create a Database Encryption Key (DEK): This key will be used to encrypt the database.

-- Example: Creating a DEK protected by the master key
CREATE DATABASE ENCRYPTION KEY
   WITH ALGORITHM = AES_256
   ENCRYPTION BY SERVER MASTER KEY; -- Or BY CERTIFICATE MyDatabaseCert / BY ASYMMETRIC KEY MyDatabaseKey
GO

Enable TDE: Apply the encryption to the database.

-- Example: Enabling TDE for a database
ALTER DATABASE MyDatabase
SET ENCRYPTION = ON;
GO

Backup Keys: Crucially, back up your master key and certificate/asymmetric key. Without these, you cannot decrypt your data if the database files are lost or corrupted.

-- Example: Backing up a certificate
BACKUP CERTIFICATE MyDatabaseCert
   TO FILE = 'C:\Certificates\MyDatabaseCert.cer'
   WITH PRIVATE KEY (
      FILE = 'C:\Certificates\MyDatabaseCert.key',
      ENCRYPTION BY PASSWORD = 'YourStrongPassword!'
   );
GO

Security Best Practice: Always back up your database encryption keys (certificates or asymmetric keys) and the master key to a secure, offline location. Losing these keys will make your encrypted data irrecoverable.

Managing TDE

Monitoring Encryption Status: You can check the encryption status of a database using system catalog views.

SELECT
    db_name(database_id) AS DatabaseName,
    encryption_state_desc
FROM sys.dm_database_encryption_keys;
GO

Disabling TDE: To disable TDE, you can use the following command. Ensure you have the necessary keys available.

ALTER DATABASE MyDatabase
SET ENCRYPTION = OFF;
GO

Moving Encrypted Databases: When moving an encrypted database, you must also move the certificate or asymmetric key that protects its DEK.

Considerations

TDE is available in SQL Server Enterprise Edition and Developer Edition. It is also available in Standard Edition starting with SQL Server 2016 SP1.
There is a performance overhead associated with TDE, though it is generally minimal for most workloads. Test your application's performance after enabling TDE.
TDE does not protect against logical attacks, such as SQL injection, where an attacker gains authorized access to the database.