Really cool librairies with source code from http://www.mentalis.org/soft/projects.qpx
Extract :
Authentication: A unified password validation library gives you an easy way to check a user-entered password against a set of validation rules. A number of rules are predefined, such as "must contain symbols" or "must be at least 8 characters", but it can easily be extended to allow for custom validation rules.
Another feature of the authentication namespace is the support for binary credentials. Binary credentials are small binary byte strings that travel with a roaming user account from computer to computer. They can be used to store passwords or other authentication data, and they will be available on any computer a roaming user logs on to.
Cryptography: The library offers support for algorithms that aren’t present in the standard .NET 2.0 cryptography namespace. Some of the included algorithms are MD2, MD4, HMAC and RC4. Additionally, the StringEncryption class makes encrypting and decrypting string based data easy.
Smart cards: The Smartcard namespaces offers a barebones framework to connect to and communicate with smart cards. It’s an excellent starting point to roll your own library for a specific type of smart card.