Depends on what one is trying to achieve. I want to verify that a message came from the right sender/is authentic, I'm not really interested in secure communications. .NET includes RSA functionality among other things which I intend to use to avoid more dependencies and potential licensing problems.
It's for product activation. Nothing too fancy, just keeping honest people honest. We are currently using a library/service for this but wish to implement our own due to licensing issues and technical problems.
OK, then SSPI or CryptoAPI is something for you to look at. Best of luck.
I apologize for not being able read your mind on what you wanted, or what you've already looked into, or that your question mark was not really a question mark. I'll try harder next time.