[From SQLServer Performance
As a Database Administrator, security is one of the most important areas to consider when it comes to protecting the databases that you support. We use various mechanisms and technologies to secure our data and databases such as firewalls, certificates, and data encryption. Having said that although we have secured our environment, questions will always be raised regarding database security. Although we have protected our databases, what would happen if someone steals the mdf file or if someone steals the backup file. Although there are few ways to control this scenario using third-party solutions up until SQL Server 2008 there has been no native way to handle this problem. SQL Server 2008 introduces a new feature that protects the database called Transparent Data Encryption - TDE which provides protection to the entire database.
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