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Schlüsselgerät 41
Hutlermühle - wanted item

Schlüsselgerät 41 (cipher machine 41), or SG-41, was a mechanical pin-wheel cipher machine that was developed during WWII by Regierungsoberinspektor Fritz Menzer as a possible replacement for the (Abwehr) Enigma machine [2]. It was manufactured by Wanderer Werke in Chemnitz (Germany) and was introduced on 12 October 1944. Although 11,000 units were ordered, only between 1000 and 2000 units had actually been delivered by the end of the war. The machine is also knows as Hitlermühle (Hitler Mill) because of the large crank at the right.
 
The machine is not based on the rather common principle of the rotor machine, like the Enigma, but on the pin-and-lug principle of the C-machines, developed by Boris Hagelin in Sweden. Similar systems were developed by Hagelin before the war (e.g. the C-38) and were used during the war by the American Army (M-209).

Although the SG-41 is clearly based on the C-38/M-209, it has been improved in a number of areas. In particular, it features very irregular wheel movements and wheel stepping in both directions, making it very difficult to break [2].

The image on the right shows a typical SG-41 machine that is in the collection of GCHQ. The machine shown here is the alphanumerical variant with 26 letters on the keyboard (A-Z). The letter J is marked in red, probably as it is used to shift between letters and numbers. When in storage, the machine is normally protected against dust and damage by a rectangular cover.
  
Photograph Crown Copyright. Kindly supplied by GCHQ [1].

According to the serial number plate that is mounted at the rear, the machine shown here has serial number 000460 and was manufactured in 1944 by Wanderer-Werke AG (manufacturer code CXO) in Siegmar-Schönau [4]. At the time, Wanderer was one of the leading manufacturers of typewriters in Germany. At the outbreak of the war, Siegmar-Schönau was a rather large industrial city in east Germany. Today, the former city is a suburb of Chemnitz (Germany).
 
The alphanumerical version of the SG-41, with the top lid removed. Photograph GCHQ [1]. SG-41 ready for use. Photograph GCHQ [1]. SG-41 with its paper compartment opened. Photograph GCHQ [1]. The six cipher wheels. Photograph GCHQ [1]. The 6 cipher wheel, the release lever and the cover. Photograph GCHQ [1]. Serial number plate. Photograph GCHQ [1]. Waffenamtabnahmestempel (acceptation stamp). Photograph GCHQ [1].

 
Controls
The exterior of the SG-41 is quite different from the Hagelin C-machines (or actually the BC-machines) on which it is clearly based. The case is higher than that of the Hagelin BC-38 and it features a large paper storage compartment at the bottom which is accessible from the front.


The machine is not driven by a motor but is hand-operated with the foldable crank at the right. At the front left of the top surface is a character counter that can be reset with a large knob to its right. Below a rectangular cover at the top are the six pin-wheels that can be released (i.e. unlocked) with a lever at the right. Once unlocked, the initial position of the wheels can be set.


The leftmost four wheels have letters on them, whilst the the remaining two are marked with numbers. In order to maximise the cipher period of the machine (i.e. the number of steps before it repeats itself), the number of steps is different for each wheel. At present, the exact number of steps on each wheel is unknownto us, but from the image above we can deduce that wheel 4 probably has 23 steps (A thru X, the 'J' is not used) and wheel 5 has 24 steps. Also note that the numbering on the rightmost wheel is not contiguous (...27 30 32 35 37...).
 
Numerical version
The SG-41 was also available in a numbers-only variant, which is shown in the image on the right. The outer dimensions of the machine are identical to the 'full' version, but the keyboard has only 10 keys that are divided over two rows.

The mechanism of the machine is nearly identical to the mechanism of a Hagelin C-38 or the American M-209. At the rear is a rotating cage with a number of bars with tabs on them. Unlike the M-209 however, the tabs on the bars of the SG-41 are not movable, just like on the Hagelin C-36 and C-37 models.
  

 
The numbers-only version of the SG-41, with the top lid removed. Photograph GCHQ [1]. The numbers-only version of the SG-41, with the top lid removed. Photograph GCHQ [1]. The numbers-only version of the SG-41, with the top lid removed. Photograph GCHQ [1]. The numbers-only version of the SG-41, with the top lid removed. Photograph GCHQ [1]. The numbers-only version of the SG-41, with the top lid removed. Photograph GCHQ [1].

 
Cryptanalysis
The SG-41 was first encountered by the codebreakers of ISK at Bletchley Park on 12 October 1944. According to former codebreaker Mavis Batey [2], they knew that the machine had six cipher wheels that moved very irregular, sometimes even backwards. Even when pure key was available to them, they were not able to reconstruct the wheel settings and the pin patterns. Such advanced features were not available on the Hagelin machines until much later with the CX-52.

Although GC&GS managed to read a few messages in depth, they were not able to solve the machine during the war. The machine remained a complete mystery to them. According to Batey, a post-war US Signal Security Report described it as a 'remarkable machine' [2].

The machine was designed by Regierungsoberinspektor Fritz Menzer, who had caused the intelligence sections ISK and ISOS at Bletchley Park problems before, with the introduction of numerous reforms and improvements to the Abwehr's ciphers. The SG-41 was intended to replace the Enigma-G that was used for the Abwehr's European communication, but by the time the war was over, it was only in use on some internal networks. Had it been implemented on a large scale, as intended, it would have posed a real problem for the Allied codebreakers.
 
References
  1. GCHQ, Photographs of two SG-41 units from the collection of GCHQ
    Photographs kindly supplied by GCHQ. 3 December 2012. Crown Copyright.

  2. Mavis Batey, Dilly, The Man Who Broke Enigmas
    2009. Hard cover, ISBN 978-1-906447-01-4.

  3. Wikipedia, Schlüsselgerät 41
    German. Retrieved January 2014.

  4. Oberkommando des Heeres,
    Liste der Fertigungskennzeichen für Waffen, Munition und Gerät

    Reichsdrückerei Berlin 1944, reprinted by Pawlas, Nürnberg, 1977.
    ISBN 3-88088-214-2

  5. Wikipedia, Siegmar-Schönau
    German. Retrieved January 2914.

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