[alblue]

There's an Enigma Machine on display in the centre of Milton Keynes this weekend https://twitter.com/alblue/status/823105879847370752, as part of the celebrations of MK's 50th anniversary. Bletchley is a suburb of Milton Keynes. http://mk50.co.uk has more if you're interested :-)

Bletchley Park has a machine on display as well, and also hosts the Bombe machine which was used to crack Enigma codes during the war. It used motors to drive copies of the rotors in Enigma in order to find a "short circuit" (where one letter coded back to itself) since this would indicate the starting point was invalid. The rotors would then tick one over and start again. The reason for so many columns was that each column of three was a different stating point - in effect, base 26 counters where each 3-digit number was started from a different number. To account for different rotor combinations and choices there were 20 or so machines built each corresponding to a different rotor choice (ABC, ABD, ABE, ACB, ACD etc). In fact it didn't just use the reflexive lookup for short circuiting; the cryptanalysts produced "cribs" which were hypothesised guesses of loops; if R-S, and S-T and T-R then this would be another short circuit option. The bombe was "programmed" with such cribs mechanically (leads plugged in to simulate connections) and then set to run. Since this process changed at midnight each day they would do the same thing day after day.

Round the corner from Bletchley Park is the National Museum of Computing http://tnmoc.org which although on the same site is a different organisation. They have the Colossus machine which was used to break the Lorenz cypher, and was the first programmable electronic computer (although the program was wired in in hardware, in effect a manual ROM).

It's well worth going to see both of these if you ever happen to be in the Milton Keynes area, or if you're passing through London - it's half an hour away by train.