You don't just 'run a cipher' - you need a mode of operation. Dr Mike Pound explains some relative to the Feistel cipher. **This video was filmed before any social distancing guidelines came into effect** EXTRA BITS: Feistel Cipher Video: Mike's code: This video was filmed and edited by Sean Riley. Computer Science at the University of Nottingham: Computerphile is a sister project to Brady Haran's Numberphile. More at