SQL Server includes a number of encryption features and capabilities that you can use to secure your systems. One of the key components of security in SQL Server is the Database Master Key (DMK). In this article, we will explore the basics of the DMK, its usage, and how you can ensure you don’t lose access to your data.

The Encryption Hierarchy

The encryption hierarchy inside SQL Server is based on the Data Protection API (DPAPI) at the Windows layer. The DMK serves as the foundation for encryption within each database. This means that you can have a separate DMK in each database on your instance. It is recommended to create the DMK in the master database for most operations.

Creating a DMK

Creating a DMK is a straightforward process. You can use the CREATE MASTER KEY syntax to do so. For example:

CREATE DATABASE MySampleDB;
GO
USE MySampleDB;
GO
CREATE MASTER KEY ENCRYPTION BY PASSWORD = 'Always*seR#@llyStr0ngP3sswo%ds5';

This code creates a database called MySampleDB and then adds a DMK with a password. The password must conform to the security requirements of the Windows host. By default, the DMK is protected by the Service Master Key (SMK).

If you need to open the DMK, you can do so by specifying the password:

OPEN MASTER KEY DECRYPTION BY PASSWORD = 'Always*seR#@llyStr0ngP3sswo%ds5';

Separate Passwords

If you want to allow multiple people to access encryption keys without sharing a single password, you can use multiple passwords to encrypt the DMK. This allows for separate access for each individual. You can add encryption with a new password using the ALTER MASTER KEY syntax:

ALTER MASTER KEY ADD ENCRYPTION BY PASSWORD = 'Jo#D3v';

To revoke access for a password, you can use a similar code:

ALTER MASTER KEY DROP ENCRYPTION BY PASSWORD = 'Jo#D3v';

Backup and Restore

The DMK is included in a database backup and can be restored like any other object. To back up the master key, you can use the BACKUP MASTER KEY command:

BACKUP MASTER KEY TO FILE = 'MySampleDBMasterKey.key'
ENCRYPTION BY PASSWORD = 'AD!ffer#ntStr0ngP@ssword5';

When restoring a database, the DMK may be decrypted by the SMK if the SMK is the same one that existed when the backup was made, and if the DMK was encrypted by the SMK. If the database was restored to a different instance, the SMK likely cannot decrypt the DMK. In such cases, you can restore the master key with the RESTORE MASTER KEY command.

It is important to manage passwords and backups of your keys to ensure the security of your data. Consider using a password manager and limiting the number of passwords you use to encrypt DMKs. Additionally, name your backups appropriately to avoid confusion.

By understanding the Database Master Key and following best practices for managing encryption keys, you can ensure the security and integrity of your SQL Server databases.