Working principle |
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Contents of this page | ![]() |
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Introduction | ![]() |
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This is an attempt to describe the working principle of the Enigma.
When developing my Enigma Simulator for RISC OS, I've searched
the Internet and read many articles and books. Most articles give a
rough description of the machine, but many important details,
necessary to deduct the algorithm, are missing. Below are my findings
in a, hopefully, readable form. If you have any questions, remarks or
additions, please do not hesitate to contact me.
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Working principle | ![]() |
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When studying the working principle of the Enigma, we have to consider
that there are in fact many different variants of this machine.
Some of the differences make it impossible to decrypt a message
that was encoded on another model. That does however not affect the working
principle as explained here. For this we study the circuit diagram
of an Enigma M3.
The keyboard consists of 26 keys, marked A-Z. Whenever a key, say Q, is pressed the wheels will be moved into a new position and a contact is closed. As a result a current will flow. The wires from the 26 keys are connected to a static wheel called the Stator or Entrittswalze (ETW). The order in which the keys are connected to the 26 contacts on the ETW varies between the different Enigma models. Leaving the ETW, the current enters the right most wheel (1) at the right hand side. The internal wiring of that wheel 'translates' this to one of the contacts of its left hand side, where it enters the next wheel, etc. Left of the rotating wheels is the Reflector, or Umkehrwalze (UKW). This wheel sends the current back into the rotating wheels, but this time the current flows from left to right, until it reaches the ETW again. From the ETW the current goes to the lamp board where the corresponding letter (W in the example) will be lit. It is inherent to this design, that a letter can never be encoded into itself. Before starting the ciphering process, the Enigma needs to be setup in a known way by both sides. This means the wheel order (Walzenlage) needs to be known as well as the starting position of each wheel (Grundstellung). In order to further complicate things, each wheel has a settable index ring that moves the contacts independant of the wheel's alphabet. This is called the ring setting (Ringstellung). To make life even more complex, some models are equipped with a plug panel (Steckerbrett) that allows letters to be swapped. Between none and 13 of such mappings may be added. If a key is not mapped (i.e. no stecker is used for that key), the letter is known to be Self-Steckered. See below for more information. |
Wheel rotation in more detail | ![]() |
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When writing the Enigma Simulator, I had to find out exactly how the
wheels are moved with each key press. First of all, the wheels are
moved into a new position, before the key is encoded and the lamp
is turned on.
Each wheel has 26 positions that we will call A-Z. The index on the wheels is engraved (either as A-Z or 1-26) along the side of the wheel. The wheels are rotated clockwise, when viewed from the ETW. If A was visible in the window, the letter B will be visible next time the wheel is moved. Each wheel has a ring that can be used to rotate the wiring independantly of the index. This can be regarded as creating an offset in the opposite direction. The notches are fixed to the index. Therefore the turnover of the next wheel, will always happen at the same letter in the window. Furthermore most models are equipped with stepping levers and notches and will therefore exhibit a double stepping feature (see below). The Enigma G series however, uses a gear and does not suffer from the double stepping behaviour. |
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The Steckerbrett | ![]() |
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The naval variants of the Enigma (M3 and M4) were equipped with
a Steckerbrett (plug board or patch panel) that would allow any
pair of letters to be swapped. If a patch cable was used beteen
G and
P, those two letters would be swapped. As we have 26 characters,
a theoretical maximum of 13 cables could be used. Most machines
however, were supplied with less (e.g. 10 or 11) cables.
Note that any number of cables can be used, from none to 13, giving a much increased number of possible permutations. As the Steckerbrett is connected between the keyboard and the ETW, the encoded letter will go through stecker mappings twice. This would prevent a letter of being encoded into itself. The picture shows single patch cable. To prevent a plug from being inserted in the wrong way, a thick and a thin pin were used. Two wires were used to cross-connect the pins of both plugs. |
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Differences in Enigma models | ![]() |
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It has been stated before that there are many different versions of the Enigma. The various models may differ in: |
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Recommended reading | ![]() |
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Thanks are due to... | ![]() |
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© Copyright Paul Reuvers. Last changed: Mon,08 Sep 2003.02:19:13 | ||
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