user18472: 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?

user13432: 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!

18472: <p>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 <a href="https://www.arduino.cc/en/Tutorial/MasterWriter" rel="nofollow">sketch found at arduino.cc</a>. </p>

<p>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?</p>
 13432: <p>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. </p>

<p>Cheers!</p>
