Computerphile just published two talks by Professor Brailsford about the WW2 German Enigma cipher machine (edit: the third and final part is now published). He explains in an excellent and entertaining way how Enigma works and how it was broken, first by the Polish en then the British codebreakers in Bletchley Park. I believe it's one of the best videos I've seen on making codebreaking understandable. A must see! The talks are made on the occasion of the release of The Imitation Game, a new movie about Alan Turing's work on Enigma. More detailed info on Enigma at these webpages.