As Sam points out, the OpenNETCF.Security.Cryptography library was donated to the OpenNETCF codebase by fellow MVP Casey Chesnut. So a good source of information about how the library is built is Casey's original article on the subject.
The library is designed in such a way that it follows the object model of the System.Security.Cryptography namespace in the full framework. A background on the Cryptographic services in .NET can be found here in the MSDN library.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.