Technical Information?

Enigma - how did it work?

The electrical diagram

Enigma I wiring diagram
Enigma I wiring diagram
(Images from www.cryptomuseum.com, used with permission.)
Enigma M4 wiring diagram
Enigma M4 wiring diagram
(Images from www.cryptomuseum.com, used with permission.)

The Enigma uses a quite simple circuit, it's basically a battery, several switches and a few bulbs with the wiring changing which switch is wired to which bulb. In the diagram, you can see that the Q key has been pressed, connecting the contact through to the battery. This is wired through the Steckerbrett plugboard (in this case, the plug is not fitted meaning it is wired direct through) to the ETW entry wheel. This was wired differently for many of the older Enigma models, but in the case of the military models simulated, it was wired directly through, A-A, B-B through to Z-Z.

This then connects through the wires connected inside each of the three fitted rotors until it arrives at the UKW reflector. Each rotor has a different wiring pattern inside. The reflector is wired in pairs with thirteen connections being redirected back into the rotors at a different position than it went in. The electrical connection then goes back through the three rotors (on a different route than before) and back through the ETW connector. The wiring then takes a second trip through the plugboard (the example shows a stecker wire pair connected which is swapping two of the wires over). This final connection then travels through the corresponding key switch (which is closed as only one key can be pressed at a time) to a bulb (E in this example) and then back to the power.

A couple of important points about wiring the Enigma up this was with the reflector was firstly that it didn't require a different wiring system to decipher a message. As long as the settings were identical, typing in one letter always gave the same result but resetting the machine and typing in that result in gave the initial letter back. The downside of this is that Enigma can never light up the same letter bulb as the key pressed (a fact that was very much used by the code breakers firstly in Poland and then at Bletchley Park to crack this machine).

Note also that as a key is pressed, but BEFORE the wiring is connected, the right hand rotor turns one position around, changing the connections and therefore which wire the electrical connection will go through. Further, at a set point, once every 26 letters, a notch on the rotor will push the next rotor along one step and so on for the third rotor. This means that typing the same letter Q will likely return a different character each time.

The rotors

Enigma rotors
Enigma rotors
(Images from www.cryptomuseum.com, used with permission.)
Enigma rotor internals
Enigma rotor internals
(Images from www.cryptomuseum.com, used with permission.)

The Enigma rotors were central to it's task of enciphering communications. Simply swapping each letter of the alphabet with another is not a very secure cipher and can easily be broken. With Engima, each time a key is pressed, one or more of the rotors moved on step changing the wiring in the scrambler which meant that even typing the same letter over and over would result in many different output letters.


Enigma rotors
Enigma rotors M4
(Images from www.cryptomuseum.com, used with permission.)

For the most common Enigma rotor, each rotor had 26 positions either numbered 1-26 or with letters A-Z around the tyre. A set of 26 sprung pins were wired from one side to a set of flat contacts on the other with each Roman numbered rotor having different wiring. Each rotor also had a ring with at least one notch set at a certain letter around the edge. The number/letter tyre and notch ring moved together and could be rotated separate to the wiring to any one of 26 positions. On the Enigma I model rotors, this was achieved by releasing a sprung-pin which allowed the tyre to rotate. For the later M1-M4 model rotors, these were replaced by a set of two levers which when both pressed together would release the tyre wheel to rotate.

Note that on the wiring listings below, the letter which is adjacent to the notch is shown as well as the letter which would be displayed in the window before the rotor would step the next one along. The pawl that drops into the notches being several steps back behind the rotors rather than on top.


Enigma I rotor wiring

Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover No. Notches
ETW ABCDEFGHIJKLMNOPQRSTUVWXYZ      
I EKMFLGDQVZNTOWYHXUSPAIBRCJ Y Q 1
II AJDKSIRUXBLHWTMCQGZNPYFVOE M E 1
III BDFHJLCPRTXVZNYEIWGAKMUSQO D V 1
IV ESOVPZJAYQUIRHXLNFTGKDCMWB R J 1
V VZBRGITYUPSDNHLXAWMJQOFECK H Z 1
UKW-A EJMZALYXVBWFCRQUONTSPIKHGD      
UKW-B YRUHQSLDPXNGOKMIEBFZCWVJAT      
UKW-C FVPJIAOYEDRZXWGCTKUQSBNMHL      

Enigma M4 rotor wiring

Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover No. Notches
ETW ABCDEFGHIJKLMNOPQRSTUVWXYZ      
I EKMFLGDQVZNTOWYHXUSPAIBRCJ Y Q 1
II AJDKSIRUXBLHWTMCQGZNPYFVOE M E 1
III BDFHJLCPRTXVZNYEIWGAKMUSQO D V 1
IV ESOVPZJAYQUIRHXLNFTGKDCMWB R J 1
V VZBRGITYUPSDNHLXAWMJQOFECK H Z 1
VI JPGVOUMFYQBENHZRDKASXLICTW HU ZM 2
VII NZJHGRCXMYSWBOUFAIVLPEKQDT HU ZM 2
VIII FKQHTLXOCBJSPDZRAMEWNIUYGV HU ZM 2
Beta LEYJVCNIXWPBQMDRTAKZGFUHOS      
Gamma FSOKANUERHMBTIYCWLQPZXVGJD      
UKW-B ENKQAUYWJICOPBLMDXZVFTHRGS      
UKW-C RDOBJNTKVEHMLFCWZAXGYIPSUQ      

Stepping mechanism

Once a key is pressed down and before the wiring is connected, a large pivoted lever is pushed down which pushes up on a bar at the rear of the machine. On this bar are mounted three sprung pawls which raise up pushing on one or more of the toothed gears at the right hand side of each rotor.

The far right hand side pawl always drops into the first rotor's drive gear and so always pushes the rotor around one step forwards. On the left-hand side of each rotor is a ring with a notch cut out (or in the case of the later M1-M4 later rotors, two notches).

The second pawl sits bridging the gap with half of the pawl resting on the notch ring and half over the second rotors drive gear. If the notch is not in position next to the pawl, then the pawl is held away from the drive gear meaning the second rotor is not driven around. Once every 26 steps though, the pawl drops into the first rotor notch and will therefore catch on the second drive gear when a key is pressed next.

The third rotor turns exactly the same as the second with it moving once every 26 positions of the second rotor turning.

Double stepping mechanism

One oddity occurs in this mechanism which causes the middle rotor to step twice on successive key presses rather than it's normal one! This is known as the double stepping anomaly. You can see an example of this occuring on Virtual Enigma by fitting rotors III, II, I in that order and setting the rotor starts to 1, 4 & 17 (or A, D and Q on the M4). If you now press a key, the fast rotor on the right steps one and the notch is in position to turn the middle rotor around too. Pressing a second key, the middle rotors notch is now in a position to turn the left-hand slow rotor but what you'll see is all three - the fast, middle and slow rotor all turn yet again meaning the middle rotor has turned twice. This can be seen in more detail on this close up video, watch as the first rotor moves setting 4 past the pawl. The first time is a single step as normal, the second time setting 4 comes past, you should see the double step anomaly on the middle rotor.

Steckerbrett

Enigma Steckerbrett
Enigma Steckerbrett
(Images from www.cryptomuseum.com, used with permission.)

The military Enigma machines (Enigma I and Enigma M1-M4) were equipped with a Steckerbrett (plugboard) mounted at the front behind a hinged panel. This board allows any two letters to be swapped by plugging a cable between two of the marked sockets. Each plug had two pins, one thick and one thin so they could only be fitted one way up. In general, 10 cables were used at a time to swap twenty of the twenty-six letters in pairs. For example, a plug between A and S would swap both an A entered to and S and an S back to an A.

The plugboard itself varied a little between models with the Enigma I having letters shown against each of the sockets while the Enigma M1-M4 changed this to numbers, but both did the same. For unknown reasons, the pins on the Kriegsmarine models (M1-M4) were around 4mm longer than the Enigma I ones making them incompatible with each other. The front wooden panel was mounted so that closing it would push all of the plugs fully home, this ensured that the connection was made correctly.

The addition of the Steckerbrett makes a huge difference on the number of possible start settings for Enigma, with 10 cables giving a multiplication factor of 150 million million times more possible starts!