.Net provides a good Cryptographic framework in the System.Security.Cryptography namepsace. But I have sometimes found it lacking for my needs (and non-intuitive at some other times).
Yesterday I needed to generate a RSA Public-Private key pair in a client-server communication, where the server generates the key pair and sends off the public key to the client, which the client subsequently uses to encrypt data being sent to the server. The server stores the Private key in the database used later to decrypt data received from the client and then send the encrypted response to it.