The Schlüsselgerät 39 (SG-39) was an electrically operated rotor cipher machine, invented by the German Fritz Menzer during World War II. The device was the evolution of the Enigma rotors coupled with three Hagelin pin wheels to provide variable stepping of the rotors. All three wheels stepped once with each encipherment. Rotors stepped according to normal Enigma rules, except that an active pin at the reading station for a pin wheel prevented the coupled rotor from stepping. The cycle for a normal Enigma was 17,576 characters. When the Schlüsselgerät 39 was correctly configured, its cycle length was 2.7 x 10 8 {\displaystyle 2.7x10^{8}} characters, which was more than 15,000 times longer than a standard Enigma. The Schlüsselgerät 39 was fully automatic, in that when a key was pressed, the plain and cipher letters were printed on separate paper tapes, divided into five-digit groups. The Schlüsselgerät 39 was abandoned by German forces in favour of the Schlüsselgerät 41.
Technical description
Note: Otto Buggisch gave the technical description of the cipher unit as part of TICOM homework. Gerät 39 is an electrically operated cipher machine. The cipher technique is derived from the Enigma cipher machine. A direct current passes through 3 or 4 wheels, with 26 positions, I, II, II, a reflector wheel U, and the again through the 3 wheels in reverse order, III II and I. Unlike the Enigma, the wheels here do not control their own movement: this is done through 3 independent pin-wheels N1 N2 and N3 with periods 21,23 and 25. The figures were distributed among N1 N2 and N2 in possibly two different configurations.
The pin wheels have a uniform motion, i.e. they move one position for every letter keyed. As for the movement of the key wheels and other details, the machine passed through different stages of development in the course of time, for which there were no specific names and which will be denoted here by a, b, c and d.
Each of three wheels moves on one place when there is an active pin at the sensing point of the relevant pin-wheel, and it only moves then. The wheels have no moveable rings on the body of the wheel, with the result that, unlike the Enigma, the initial position of the body of the wheel is determine absolutely, at the same time as the clear message setting. The reflector wheel is pluggable like the reflector wheel D on the Enigma; it can be quickly exchanged for the second reflector wheel with prepared reflector plugging.
Wheels I, I and III, whose wiring now correspond exactly to those of the Enigma, have adjustable rings; they can be moved around the body of the wheels and have a fixed pin, which, by analogy with the Enigma, is to be called the turn-over notch, although mechanically it is not so made. Opposite the wheels I and II are two sensing points which pick up the turn-over notch as it passes. U is pluggable as in a). In addition there is between the point of input and I, a stecker S like the Enigma stecker. The following two methods of working are possible:
Working on own wiring: N1 N2 and N3 are given a certain pin arrangements, there being, it is true, certain limitations to the number of activating pins. Wheel I moves as under a) above. For wheel II there are the following 3 causes of movement: An active pin at the sensing point of N2 causes II to move on one place as in a) above When the turn-over notch on the ring of I comes to the sensing point, II is caused to move on when the next letter is keyed (as with Enigma). When the turn-over notch comes to the sensing point of II, II turns on one place when the next letter is keyed (at the same time as III, as with the double step on Enigma)
If any of these three causes of movement take effect simultaneously on II, it nevertheless only moves on one place. There are three causes of movement for wheel III. An active pin at the sensing-point of N3 causes III to move to one place, as in the first a) above, When the turn-over notch on the ring of II comes to the sensing point, III moves on one place when the next letter is keyed.
Just as in the case of II, if the two causes of the movement for III operation simultaneously the combine to produce one stop.
Working on Enigma wiring. All the pins of N1 are set at active, the pins of N2 and N3 all remaining inactive. Then the movements identical with that of the Enigma. As all other factors also agree with the corresponding ones on the Enigma interchangeable working between both machines is possible.
A sensing-point is also provided opposite wheel III. If the turn-over notch on the ring of III is touched by it, then I turns on one place when the next letter is keyed. If this movement coincides with the step caused by N1, this again results in the single step. Thus the possibility of interchangeable working with the Enigma remains. In addition, the machine now gets a fourth wheel, which is placed between III and U and does not move on when a key is touched. It corresponds to the fourth wheel on the Naval Enigma and is used for interchangeable working with this machine.
In the summer of 1944, Karl Stein of OKW/Chi told Buggisch that the reciprocal influencing of the wheels was to be altered in some way. Buggisch could not remember the details but nothing fundamental on the principle of the machine described under c) was changed. Interchangeable working with army and naval Enigma remained possible.
Investigations in Periodicity In the case of model first a) above, the question of periodicity is elementary, there are 262=676 pure periods of the length
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