For a project I'm using 2 Arduino Uno's, where Arduino#1 (master) sends raw data to Arduino#2 (slave). The slave should receive and encrypt the data and afterwards send the encrypted data back to the master. I'm building my project from the sketch found at arduino.cc.
How can I make the slave send back data, if he's the only one listening on incoming data, should both arduinos be listening to each other, and how to achieve this?