Just wanted to post a library here which I wrote to send encrypted data across a radio based telemetry link. You can use it over any serial port though.
It's in the hope that someone finds it useful.
Supports AES-128, AES-192, AES-256, 3DES and Unencrypted at the moment with sub-channels.
The README in the project should help.
Finally the project is here.
PS: I used it on Nano and Mega but should work with others too.