The Enigma was used solely to encipher and decipher messages. In its standard form it could not type a message out, let alone transmit or receive it. From the cipher operator's point of view, it consisted of first a keyboard of 26 letters in the pattern of the normal German typewriter, with no keys for numerals or punctuation. (See diagram below.) Behind this keyboard was a "lampboard" of 26 small circular windows, each bearing a letter in the same QWERTZU pattern, which could light up, one at a time, from bulbs underneath. (The model with an A-Z keyboard, shown in several books on the Enigma, is a Polish-French replica, not an actual Enigma machine.) It measured about 13.5" x 11" x 6", and weighed about 26 lbs.
Behind the lampboard is the scrambler unit, consisting of a fixed wheel at each end, and a central space for three rotating wheels. The wheel to the right of this space is the fixed entry or plate (Eintrittwalze) carrying 26 contacts round its left side, ultimately connected to the keys of the keyboard in ordinary alphabetical order. To the left of the space is the reversing wheel (Umkehrwalze), which scrambles the current it receives and sends it back by a different route from that by which it came. This wheel too has a circle of 26 contacts.
The three central wheels were selected from a box of five. Monthly orders specified a new choice every day, as well as their relative order in the machine, e.g., V-I-III or II-IV-I, etc. Each of these rotating wheels has a circle of 26 spring-loaded terminals on its right side and 26 flat circular terminals on its left, so as to provide an ever-changing series of connections as it revolves. Each contains a different internal wiring and carries the letters A-Z or the numbers 01-26 round its inner ring, which can be turned and locked in any setting before the wheels specified for a given day are inserted into the machine in the prescribed order, though they can still be turned bodily through slits in the inner lid, and the letters A-Z can be read one at a time in the window beside each slit. The specified setting of ring against wheel was called the Ringstellung.
Each time a letter key is pressed, the right wheel moves on one of its 26 places. Once during every 26 moves, at the "turnover position" on the right wheel, the middle wheel will also move on one place; and when the middle wheel reaches its own turnover position it moves on again when the next letter is keyed, together with the left wheel.
Finally, the vertical front of the Enigmas used by the Armed Services contained a "plugboard" with 26 pairs of sockets, again in the QWERTZU pattern. These could be connected by twin-cable leads—for example, coupling C to P, M to Z, J to S, and so on; but some sockets, usually six, were left unconnected. They were said to be "self-steckered." Stecker is a plug; Steckerbrett (usually called "steckerboard" at Bletchley) is a plugboard.
Each time the cipher clerk keyed a letter, the right wheel moved on mechanically one place and, as explained above, from time to time the center and left wheels also moved. As each new letter (e.g., P) was keyed, the current, normally provided by an internal 4.5 volt battery—although an outside power source could be used—flowed from a terminal under that key to a socket (e.g., P) on the plugboard. From there it travelled via a lead to another socket (e.g., L), or, if the first socket was self-steckered, it stayed as P. Either way, it ran to the entry wheel, which did not alter it, through the pairs of terminals on all central wheels—each of which normally altered it again—to the Umkehrwalze or reversing wheel (with another alteration) and back through different circuits in all three wheels (hence still further alterations), out unaltered through the entry wheel, and back to the plugboard. Here its course again depended on whether that socket was self-steckered or cross-steckered; either way, it finally reached the lampboard and lit a bulb (e.g., W). Although the process, involving up to nine changes on the standard three-wheel machine, has taken some time to describe, it naturally took place virtually instantaneously. And it must be remembered that the moving on of at least one wheel, for every new letter keyed, introduced a new set of circuits for each new letter.
It is important to note that, if you press any key (e.g., B), any other letter may light up (e.g., T); but if you continue to key letter B, the lampboard may give, say, P, F, O, J, C..., but never B. The sequence will repeat only after 16,900 (26 x 25 x 26) keyings, when the inner mechanism returns to the same position. Messages were limited to a maximum of 250 letters to avoid this recurrence, which might have otherwise helped us.
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 í and í have been added, there is no í. 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.