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How the Enigma Works
Part 2 | Back to Part 1
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.
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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