The Enigma cypher machine used by the German military in World War II is still a tough nut to crack today. The total number of ways it can be configured for every letter is around 150 million million million. That's enough to keep it beyond the reach of all but the most determined of brute force attacks.
So how were the late Alan Turing (whose 100th birthday is being celebrated in academic circles this June) and his fellow Bletchley Park code-breakers able to crack the Enigma and provide the Allies with such priceless intelligence?
To begin with, Google Internet evangelist Vint Cerf says, there was a lot more to it than raw computational genius.
"It's not entirely a result of the hardware. It's also people making extraordinary guesses about what might have been encoded, the formats, date and time of transmission; all these other things factored into the tactics for trying to break a particular message," he says.
Today's hackers would instantly recognise some of the British techniques for hacking German messages as social engineering - on a grand scale.
One British trick called for the Royal Air Force to drop sea mines at a location where they would be sure to be noticed by the Germans - which would invariably prompt a notification that "the British have mined grid coordinates x,y." The British could then work on that message with the knowledge that the German message would include those coordinates, giving them a substantial leg-up.
"These are called chosen plaintext attacks, and they are perniciously powerful. They're the sort of thing that any cryptographer really hates," Cerf says with a laugh.
Nevertheless, the advances in computing technology - particularly Turing's Bombe, created to automatically crack Enigma messages - were staggering, and also essential to the code-breaking effort.
"It's incredible what people did during World War II," says Cerf. "Given the crappy computational capabilities they had, and the very limited hardware that they could build, it's amazing what they got away with."