Phone-To-Server communication encryption in Java

Question

Hi,

This goes along with another thread, here: How to implement Java 256-bit AES encryption with CBC

Mainly, trying to get AES encryption on a phone using Java.

My question here is how to handle the encryption key. I don't know if I should store the key, hash the key and use that, or do a public-key-encyption scheme. I would rather have a way to not do an initial message from the server to the phone to communicate a key. I want the phone to be ready to encrypt and the server waiting for an encrypted message. To implement something like a key-sharing algorithm, I would have to modify our server application which is not very desireable. It's not impossible, but I'm going for code-reuse here =).