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Probemaschine
Early Enigma prototype

Enigma-Probemaschinen (English: Enigma test machines) were early prototypes of a rotor-based cipher machine, developed from 1918 onwards by German inventor Arthur Scherbius. They led to the development of two separate lines of the Enigma cipher machine: (1) the Glühlampen­chiffrier­maschinen (glow lamp cipher machines) and (2) the Schreibenden Maschinen (printing machines). As far as we know, there are no surviving examples of Scherbius' early prototypes of the Enigma.


Layout
The image below shows what the control panel of the Probemaschine might have looked like, based on the drawings in the patent description [2]. At the left are 25 keys arranged in a 5 × 5 matrix, printed with the letters A-Z (note that the letter 'J' is omitted). At the right are 25 flat-faced lamps, also arranged in a 5 × 5 matrix. The lamps represent the output of the cipher.


It is possible that the lamps were covered by a film on which the letters A-Z were printed, but this is not certain. Move the mouse over the drawing to see what it may have looked like in that case. In the drawing above, the user has pressed the letter 'G', whilst the letter 'T' lights up. The reason for the missing letter 'J' is probably that with 25 letters the keys can be nicely arranged in a square matrix, which is not possible with 26 letters. It was a common arrangement that was also used by other cipher systems. In the text, the letter 'J' can easily be replaced by the letter 'I'.


History
2-rotor machine
The first prototype was made in April 1918 and had two rotors with 25 contacts each, resulting in 252 = 625 different alphabets. It was clear to Scherbius however that this was weak, and that the strength of the cipher had to be increased by adding more rotors. During a demonstration for the German Navy on 10 May 1918 he gave the following examples for different numbers of rotors [3]:

Rotors Alphabets Notation  
2 625 252  
7 6,103,515,625 6.1 × 109  
10 95,367,431,640,625 9.53 × 1013  
12 59,604,644,775,390,625 5.96 × 1016  
7-rotor machine
Shortly afterwards, at the request of the German Navy, Scherbius demonstrated another prototype with 7 rotors. Although the demonstration went well, he soon experienced contact problems, that were probably caused by oxidation and mechanical friction between the rotors [3]. It caused Scherbius to drop his initial goal of using a large number of rotors, and settle for just three or four, which resulted in the following – less astronomical – number of different alphabets:

Rotors Alphabets Notation  
3 15,625 1.56 × 104  
4 390,625 3.9 × 105  
Alternative improvements
In the following years, Scherbius and his staff would try to find other ways to strengthen the cipher, for example by adding additional rotors to choose from, swapping the rotor positions, adding a reflector and adding a plugboard, altough not all of these had the desired effect.

The reflector – known as the Umkehrwalze or UKW – greatly simplified the construction of the machine and its operation, but reduced the strength of the cipher. Likewise, the final design of the plugboard – known as the Steckerbrett – made the construction of the machine much more complex, but added very little extra security, despite the huge number of possible settings [3].


Circuit diagram
Below is the simplified circuit diagram of the first prototype, based on the information given in the original patent [2]. To show the principle, only the letters A-F are shown here. At the top are the two rotors (I, II), located in between two contact discs (ETW1, ETW2). At the bottom are the keys A-F, used for input, and the lamps A-F, used for output. At the center is a commutator that can swap the wiring of ETW1 and ETW2. It is used for selecting between CIPHER and DECIPHER.


The diagram above shows the Probemaschine in CIPHER mode. When pressing the letter 'F', the current flows from the battery through switch 'F' on to the com­mu­tator. From there it flows through ETW1, rotor I, rotor II and ETW2, until it enters the right side of the com­mu­tator. From the commutator the current flows to the lamp panel where in this case the letter 'E' lights up.

Move the mouse over the diagram to see what happens when the device is switched to DECIPHER.


References
  1. Wikipedia (Germany), Enigma-Probemaschine
    Visited 16 Aprl 2023.

  2. German Patent DE416219, Chiffrierapparat
    Arthur Scherbius, 23 February 1918.
     Other Enigma-related patents

  3. Olaf Ostwald, Cryptographic design flaws of early Enigma
    5 April 2023.

  4. Claus Taaks, Scherbius and Enigma History
    Personal correspondence, April 2023.
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