For learning purposes, I'm creating a chat application where the connections are done via SSL/TLS, and the messages are encrypted using AES-CBC-256 and the AES keys are encrypted with RSA-2048.
The AES key is randomly generated (AesProvider.GenerateKey()) per user per session (which means one key for every person an user is chatting with) and the IV is randomly generated (AesProvider.GenerateIV()) by passing in the key generated, each time a message is created (before being sent). On the RSA side, I'm generating a secure random session name to store the private keys generated in containers, and sending out the public key. I'm also using the same model (one key pair per user per session) as in AES.
I should also state that I'm using HMAC-SHA512 to hash the messages and sending the HMAC key encrypted using the same public key that the AES key/Iv gets encrypted with. Since I've read that it doesn't need to be regenerated often, I'm planning on regenerate the HMAC key every 5000 or 10000 calls.
Questions: 1) Should I be creating only one RSA key pair per user and use it for all sessions, or is it good how it is right now? 2) Is using the same AES key and only changing the IV like explained above considered secure?