The number of rotors was limited by the keyboard effort. All those spring-loaded contacts between the rotors make advancing the rotors hard. Enigmas have a huge key travel, almost an inch, to provide extra leverage. You can't actually "type" on one; you push the keys down firmly, one at a time. Somebody has to write down which lamp came on, anyway, so that's not the limit on speed.<p>The keyboard-driven rotor machines didn't scale up well. Beyond three rotors, motor drive was usually needed, although the cute little M-209 used a wheel and lever for input, rather than a keyboard. The next step up was the Lorenz SZ-40/42, a six-rotor machine set up as a peripheral for a Teletype. That was cracked with the Colossus electronic key-tester.<p>The Germans built the T-52 Geheimschreiber, with 10 rotors, but that had a built-in Teletype machine and was too heavy to be portable. The Allies didn't crack that one.<p>That was the original limit on key length. The longer the key, the bigger and heavier the machine.
The Enigma is a neat machine. I wrote a Java iButton applet that emulated one and in doing research for that came across the rotor patent (<a href="https://www.google.com/patents/US3984922" rel="nofollow">https://www.google.com/patents/US3984922</a>) this patent was <i>filed</i> in 1944 but it <i>issued</i> in 1976. October 12th to be precise. One month before the Data Encryption Standard (DES) came out as a FIPS standard. How Enigma worked had remained classified for decades after the war ended, that is a pretty strong endorsement for this remarkable machine.
Like most, I always thought that the allies were desperate to get their hands on these machines during the war.<p>I was lucky enough to get to play with one of these things when a lecturer from the Bletchley Park foundation toured here some years back. She gave a fascinating and riveting talk on the history of the device.<p>I was surprised to hear that these were actually commercially available before the war, and that the allies had quite a few of them in store already. The crucial thing they were after were the booklets with the daily rotor and plugboard settings - far more valuable than the machines themselves.
All things considered, 84 bit encryption is pretty impressive, considering DES was still considered "okay" even into the late 1990's.<p><a href="https://en.wikipedia.org/wiki/Data_Encryption_Standard" rel="nofollow">https://en.wikipedia.org/wiki/Data_Encryption_Standard</a><p><a href="https://en.wikipedia.org/wiki/Deep_Crack" rel="nofollow">https://en.wikipedia.org/wiki/Deep_Crack</a>
It really was quite a remarkable device, and if I remember correctly offered much more security than was strictly "necessary" at that stage of the crypto wars, though of course we know how all that particular story ended.
There's an Enigma Machine on display in the centre of Milton Keynes this weekend <a href="https://twitter.com/alblue/status/823105879847370752" rel="nofollow">https://twitter.com/alblue/status/823105879847370752</a>, as part of the celebrations of MK's 50th anniversary. Bletchley is a suburb of Milton Keynes. <a href="http://mk50.co.uk" rel="nofollow">http://mk50.co.uk</a> has more if you're interested :-)<p>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.<p>Round the corner from Bletchley Park is the National Museum of Computing <a href="http://tnmoc.org" rel="nofollow">http://tnmoc.org</a> 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).<p>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.
After reading to the end (very enjoyable),be sure to find out what weaknesses Bletchley Park exploited to break the encryption:<p><a href="http://users.telenet.be/d.rijmenants/en/enigmauboats.htm" rel="nofollow">http://users.telenet.be/d.rijmenants/en/enigmauboats.htm</a>