Secure Socket Library

The .NET class library offers SSL support when you connect to an HTTP server, but unfortunately it does not offer SSL or TLS support for other Internet protocols.
There are several 3rd party libraries available that offer SSL and TLS support for .NET applications, but they cost a lot of money and they are typically closed source.

The Mentalis.org SecureSocket library is a free and open source implementation of the Secure Socket Layer (SSL) protocol version 3 and the Transport Layer Security (TLS) protocol version 1.
Support for the Private Communication Technology (PCT) protocol and SSL version 2 protocol was present in the older versions of the Security Library, but has been removed in the latest release. If absolutely necessary, you can still download the older version of the Security Library for SSL2 and PCT support.

As its name implies, the SecureSocket library implements a SecureSocket class that behaves like an ordinary Socket class and transparently encrypts and decrypts data sent through it. If you are familiar with the Socket class, you'll find that it's almost trivial to work with the SecureSocket implementation.
In addition to the SecureSocket class, the library also ships with a SecureTcpListener class, a SecureTcpClient class and a SecureNetworkStream class.

The SecureSocket library is now a part of the Mentalis.org Security Library. You can download the Mentalis.org Security Library over here.
For more information about the SecureSocket library, check out the Security Library documentation.