The breaking of the Enigma Code is one of the most interesting stories in the history of computing—but it's not necessarily easy to get your head around. Fortunately, this video provides an amazingly straightforward explanation of how the code was cracked.
James Grime, a mathematician from Cambridge University, explains how Turing and his coworkers managed to defeat the German encryption. It all stemmed from the fact that the code contained a subtle flaw: a letter was never encrypted in such a way that it was represented by itself once coded. From there, Turing was able to work out a solution.
Of course, the problem was that the code changed every night. So Turing built the Bombe: a machine that could crack the code, every morning, in just 20 minutes. At this point, though, I'll hand over to James—who can describe the whole process far more elegantly than I could ever hope to. [YouTube]