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How the Enigma Works
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In choosing a basic set-up for the machine, there was a choice from the 60 possible wheel orders, the 17,576 ring-settings for each wheel order, and over 150 million million stecker-pairings (allowing for six self-steckered letters). So the total number of daily possible keys was about 159 million million million. In each of these configurations, the machine had a period of 16,900 (26 x 25 x 26) keyings before the mechanism returned to its original position. But there were weak points. The Enigma is simply a swapping machine of an advanced type. All Enigmas of the same model, set up in the same way, will produce identical swaps. In any position where keying B gives T, keying T will give B. And keying B can never give B.

Although it was possible for one cipher clerk to carry out all the tasks of the enciphering procedure himself, this would have been a lengthy and confusing process; normally it called for a team of two. The cipher clerk would look at his signal text, which might begin Panzer ("tank(s)"). Typing P might give M on the lampboard; his Number Two would read this and write it down—and so on through the message. The radio operator would then transmit the resulting enciphered signal. But first the machine had to be properly set up.

Every month the operating instructions specified daily or more frequent changes to several variables. A typical daily "key" gave the clerk instructions for the first three steps of the enciphering procedure.

The wheel order (Walzenlage): the choice and position of the three wheels to be used (e.g., I-V-III).
The ring-setting (Ringstellung) of the left, middle, and right wheels (e.g., 06-20-24 denoting FTX).
The cross-plugging or "steckering" (Steckerverbindungen) (e.g., UA PF etc.).

The cipher clerk would set up his machine accordingly. Until the end of April 1940, he then continued as follows:

He turned his three wheels to a position chosen at random, the "indicator-setting" (e.g., JCM).
He twice keyed his own randomly selected choice of text-setting, or "message-setting" (e.g., BGZBGZ).
This came out as the "indicator" (e.g., TNUFDQ).
He set his wheels at BGZ and keyed the clear text of the message, thus obtaining the enciphered text, letter by letter.

The message as transmitted included four elements, as follows:

The preamble, transmitted in clear before the message itself, showing call-sign, time of origin, and number of letters in the text; this was followed by his chosen indicator setting (e.g., JCM) (No. 4 above).
A five-letter group comprising two padding letters (Füllbuchstaben) followed by the three-letter "discriminant" (Kenngruppe), e.g., JEU, which distinguished various types of Enigma traffic and showed which of many "keys" (sets of operator instructions) were being used. The latter were known at Bletchley by cover-names such as Kestrel, Light Blue, etc.
The six letters of the "indicator" TNUFDQ (No. 6 above).
The enciphered text of the signal, in five-letter groups.

Once the signal had been transmitted in this form, and the text handed to the receiving cipher clerk—whose wheels would already comply with the same daily instructions Nos. 1-3—he would duly move his wheels to JCM (No. 4 above), key TNUFDQ (No. 6), and read the reciprocally enciphered result BGZBGZ (No. 5.) He then turned his wheels to BGZ and deciphered the text by keying it out, with his Number Two noting each letter in turn.

After 1 May 1940 this procedure was changed. Presumably the German cryptographic authorities had belatedly recognized that the double encipherment of the text-setting represented a security risk which far outweighed the advantage of the double-check it provided. From that date the random choice of text-setting (e.g., BGZ as in No. 5) was keyed only once, giving TNU instead of TNUFDQ.

Lighted 'y'

The reader should also bear in mind that the foregoing description of mechanism and procedure applies only to the standard Enigma used by the German Army and Air Force. The Navy provided three special wheels in addition to the five Army-Air Force wheels, and thus had a set of eight to choose from. On 1 February 1942 they added an extra settable wheel, next to the Umkehrwalze, resulting in the M4 model, often called the "4-wheel" Enigma. The railways, police, and post office used older Enigma models, while the Abwehr used an advanced but unsteckered one, and a different enciphering procedure, with a Grundstellung specified for each day's settings, instead of allowing a random choice. Certain other unusual models had a 28-letter keyboard and wheel system. It seems clear that the "29-contact rotor" (wheel) suggested for this machine could not have existed (see C. A. Deavours and L. Kruh, Machine Cryptography and Modern Cryptanalysis. Norwood, Mass.: Artech House, 1985, 96-7.) The Enigma was essentially a reversing machine with an even number of wheel contacts, and although Ä and Ü have been added, there is no Ö. The 29-letter keyboard of this machine is thought to have had one letter, X, which bypassed the wheels and always gave the letter X.

Excerpted with permission from Codebreakers: The Inside Story of Bletchley Park, edited by F. H. Hinsley and Alan Stripp (Oxford: Oxford University Press, 1997). At the time of publication, Stripp was Director of Cambridge University Summer Schools on British Secret Services.

Photos: (1) Corbis/Dorling Kindersley Limited, London

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