18 February, 2006

Herivel Tip

Bletchley Park has announced a forthcoming lecture by WWII veteran codebreaker John Herivel, a Hut 6 mathematician who, within weeks of arriving at BP, had come up with a nifty bit of lateral thinking to help solve Enigma. Dubbed the Herivel tip or Herivelismus, it relied on Enigma operators taking a shortcut and not randomising the rotors after having set up the machine. If you're interested, I wrote up the details in a Wikipedia article. It was Hut 6's lifeline for a few months in the Summer of 1940 after the Germans had changed their indicating procedure, obsoleting the Polish techniques then in use.

Herivel's tip reminded me of combination locks, of the type with rows of dials of digits. On university campus, I've noticed that many people in a hurry don't really scramble their combination locks (for cycles, normally) very thoroughly -- maybe a quick flick of the dials with the thumb, or something of that sort. As a result, the state after a half-hearted scramble still reveals information about the secret combination. I did some tests (on a lock of my own, of course), and if you observe several of these states, and you have a reasonably accurate model of what weak scrambling method is being used, you can whittle down the possibilities pretty quickly. Still, a good old-fashioned pair of bolt cutters is less hassle...