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 

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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