How do you compute a massive number raised to the power of another huge number, modulo something else? Dr Mike Pound explains the super-quick square & multiply algorithm. Numberphile's Witness Numbers video which inspired Mike: https://www.youtube.com/watch?v=_MscGSN5J6o https://www.facebook.com/computerphile https://twitter.com/computer_phile This video was filmed and edited by Sean Riley. Computer Science at the University of Nottingham: https://bit.ly/nottscomputer Computerphile is a sister project to Brady Haran's Numberphile. More at http://www.bradyharan.com/