The Man Who Knew Too Much: Alan Turing and the Invention of the Computer (Great Discoveries) by David Leavitt

Author:David Leavitt
Language: eng
Format: mobi, epub
Publisher: W. W. Norton & Company
Published: 2006-11-16T22:00:00+00:00

The Enigma Machine, employed by the Germans to encrypt classified and sensitive messages during World War II. (HultonArchive/Getty Images)

Inside each rotor a mass of wires connected the contact points on the right face to those on the left. This wiring, though arbitrary, was fixed; that is to say, even though the rotors could be placed in different orders, all the rotors in all the Enigma machines in the system were wired alike. This insured that rotor 1 on the sender’s Enigma would have wiring identical to that of rotor 1 on the recipient’s Enigma. And since the rotor order was one of the elements fixed in advance, there was no chance of cross-wiring.

A series of switches connected the rightmost of the rotors to the keyboard. In the commercial Enigma, the letters on the keyboard were linked up with the contact points on the first rotor in the same order as that found on the keyboard; in the military Enigma, however, the wiring had been changed, and one of the first challenges the code breakers faced was to figure out what the new order was. Because the top row of letters on a German typewriter reads QWERTZUIO (as opposed to the QWERTYUIOP found on American typewriters), Dilwyn Knox, one of the first Englishmen to take on the Enigma, referred to the mysterious new letter order as the “qwertzu.” Though Knox feared that this order might prove so arbitrary as to defy analysis, to his great surprise, a group of Polish cryptanalysts led by Marian Rejewski quickly determined that in the military model of the Enigma, the Germans had simply connected the letters on the keyboard to the contact points on the first rotor in alphabetical order. This was the first of several instances in which German laziness and lack of imagination ended up aiding the code breakers in their effort to defeat the machine.

When a letter was typed into the Enigma, current flowed from the keyboard into the rightmost rotor, which then shifted one position, thus changing the letter’s identity. The current continued through the other two rotors, with a substitution occurring in each position. Next the current entered a “reflector,” a half-width disk at the left end of the machine with contacts only on its right side. The reflector connected pairs of letters, replacing the incoming letter with a second one, which would then be sent back through the three rotors for another series of substitutions. Its function was to guarantee that no letter typed into the Enigma could be enciphered as itself; it was also responsible for the Enigma’s property of being able to serve as both an encipherment and a decipherment machine.

A last element—incorporated into the military Enigmas—was a “stecker” board, rather resembling an old-fashioned telephone switchboard and located at the base of the machine, with twenty-six jacks (or “steckers”) into which cables could be plugged. On these machines the positions of the steckers also had to be agreed upon in advance. Steckered pairs of letters would be

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