Using symetric encryption is very suboptimal because you have to exchange the keys which MUST be protected (the one who has the key can decrypt everything). It's very fragile.
The only safe way is to use RSA. As you know the public key is public and can be exchanged in any way you like. Messages can only be encrypted with the privat key which is kept secret.
Agraham has an library which supports RSA. App2App communication works safe with it. The problem is when you want to communicate with a server. Most hosters don't allow you to use OpenSSL or other libs, except you pay (a lot of) money for it. Even if you buy it, I did not find a way to make the keys byte compatible.
However. Usually you use php for the server part. So there is a need to have RSA here without a lib or other components. I've finished the B4A logic yesterday. In the next weeks I will develop the php script and write a tutorial.