Correction : as oodles of commenters have pointed out, the clock face should go from 0 to n-1. Also, worth reminding people that Mike has simplified the notation in this video (as he mentions). Mike explains the mathematics behind one of the most important pieces of computer security. (Simplified version with colour mixing analogy linked below) Secret key Exchange (Colour Mixing) Video: 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