Send AND Receive through i2c / set Arduino as master AND slave 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?
The I2C protocol is both read and write. Master/Slave depends on who initiates the communication. Typically the master will issue a read command to the slave to retrieve the results. 

Cheers!