What are you looking for beyond what the .NET Compact Framework provides in
System.Security.Cryptography[1] (which provides quite a bit of supported
functionality)? Also, are you using .NET CF 1.0 or 2.0 ?
We're using the C# port of the BouncyCastle library
(http://www.bouncycastle.org/csharp/) and are quite happy with it (awesome
functional range and unit tests). To compile it for .NET CF you have to make
some small modifications (i.e. removing Serializable attribute references).