Since the inception of SQL Server 2005 data encryption and cryptography functionality has elevated to a new level, this wasn't the case until SQL 2000 version.  

The overall security improvements within SQL 2005 is mainly with 'certficate' authentication, the 3 levels are:

  • Authentication – subjects (persons, devices, services) are granted access to the server (e.g. through HTTP endpoints, as database principals) after a valid certification path has been verified;
  • Module signatures – procedures, functions, triggers and assemblies can be signed with certificates; and
  • Encryption – data and symmetric keys can be encrypted using certificates.
So within this encryption functionality "Symmetric keys" play vital role and recommended for data encryption by the experts (experience too) on the aspects of providing better performance when encrypting and/or decrypting data. This is different in comparison of asymmetric keys and certificates, as the platform provided for cryptographic functions are not entirely problem-free or atleast low performance as it grows with the sheer size of data you need to manage. As per the documentation "Symmetric keys in SQL Server 2005" are matched with the following cryptographic algorithms: DES, TRIPLE_DES, RC2, RC4, RC4_128, AES_192 etc. and again these are very specific on the Windows operating systems such XP and 2003.

The overall funcationality of digital signing of modules and cryptographic objects in authorization & authentication are documented within the Books Online, on top of that I recommend to review the information from MSDN & Technet experts blogs such as: Raul Garcia, Jeff_Jones & Laurentiu Cristofor blogs.