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Enigma G
Zählwerk Enigma G31 - wanted item

The Enigma G was developed in 1931 as one of the successors to the earlier Zählwerk Enigma (model A28) of 1928. It features a cog-wheel driven wheel mechanism with irregular stepping and a crank to wind it back and forth. The official model number of the machine was G31 but it is commonly referred to as the Enigma G, as the serial numbers start with the letter 'G'. The official designator is Ch.15a, whilst the codebreakers of Bletchley Park called it the 11-15-17 machine (after the number of notches on each wheel). It is also known as the Abwehr Enigma (see below).
The machine is different from all other Enigma models, not only because of the cog-wheel driven mechanism, but also because of its smaller size, its smaller wheels, and its sloped lamp panel with a large lever sticking out at the top centre, just behind the cipher wheels.

The image on the right shows the Enigma G with serial number G-312 that is part of the Bletchley Park collection. The machine was stolen from the museum in 2000 but has since been returned and is now on public display again [1]. It was photographed by Crypto Museum in 2004 [2]. 1
Close-up of Enigma G

The G31 was probably an attempt to make a smaller —more portable— version of the Zählwerk Enigma. The wooden transport case is smaller than any of the earlier ones and even the cipher wheels are smaller, making them mechanically incompatible with any other Enigma model. The machine uses a much smaller battery that is mounted below the switch assembly at the top right, but may also be powered by an external source, for example by a transformer. A crank, that is normally stored inside the top lid of the wooden case, can be inserted into a hole at the right side of the machine, allowing the cipher wheels to be stepped manually when making corrections.

Three different versions of the Enigma G31 were made. The most common one was Ch.15a, the standard version that was supplied, for example, to the German Abwehr and the Dutch Navy. The Ch.15b had a printer socket on its left hand side and the Ch.15c was a special version with an alternative Steckerbrett (plug board). So far, the Ch.15c has never been rediscovered. We are indebted to Frode Weierud [10] for his tireless support in recovering the history of Enigma G.
  1. The quality of the photographs of the Enigma G-312 is somewhat sub-standard as they have been scanned from analogue negatives. At the time we didn't have a digital camera.

Enigma G at Bletchley Park Close-up of Enigma G David hamer (rear) and Marc Simons (front) discussing Enigma G Marc Simons investigating the Enigma G-312 Wheels of the Enigma G-312 Enigma G entry wheel (ETW) and counter Enigma G reflector (UKW) Driving cog-wheel of the Enigma G

The diagram below shows the various features of the (opened) Enigma G31. The upper half of the image shows the inner side of the sloped top lid. The lower half shows the machine with (from top to bottom) the cipher wheels, the lamp panel and the keyboard. Note the complex cog-wheel driven wheel stepping gear that is a typical feature of the Enigma G and the Zählwerk Enigma.

This machine does not have a simple battery compartment like the other models, but has a much smaller one that is hidden below the power switch assembly at the top right. After releasing the two large thumb-screws, the switch assembly can be removed and the battery can be accessed. 1
  1. Thanks to Glenn Miranker for spotting the battery compartment in 2015.

The case, and hence the wooden box, of the G31 are smaller than that of an Enigma D or an Enigma I. This was probably done to make it more portable. The outer dimensions of the wooden box are 25x27x16.5 cm. The following differences with other machines have been recorded:
  • Smaller case
  • Smaller wooden transport box
  • Smaller cipher wheels (Ø 8 cm rather than 10 cm)
  • Cog-wheel driven stepping gear
  • Gear locking lever (protruding the top lid)
  • Crank (stored inside the lid of the wooden box)
  • Character counter
  • Smaller battery
  • Sloped lamp panel
  • Letter and symbols on the keyboard (and the lamp panel)
  • Absence of a green filter inside the lid of the wooden box
  • Absence of a lamp test socket
Cipher wheels
The wheels of the Enigma model G31 are smaller than those of all other Enigma machines. The largest diameter is approx. 85 mm (3.5 inches), whereas the normal wheels are 100 mm (4 inches). In order to accomodate the 26 spring loaded contacts, they are arranged in a zig-zag pattern. The contact pads at the other side of the wheel have the shape of a tear-drop.

The wheels of the initial Zählwerk machine (left) and the Enigma model G31 (right)

  • Ch.15a - standard version
    This is the standard version of the machine. Most Enigma G machines that have survived, are of this type. A good example is the Enigma G-312 that is part of the Bletchley Park collection, and the G-260 that was found in Argentina towards the end of World War II.

  • Ch.15b - with printer socket
    This version is functionally identical to the standard model (Ch.15a) except for the fact that it has a circular socket at the left, allowing another machine (such as the Enigma H) to be used as a printer attachment. A good example is the Enigma G-111 (see below).

  • Ch.15c - with plug board
    According to the personal notes of Enigma-developer Willy Korn, this version had a plug board that was different from the plug board of the Enigma-I, as the latter was used exclusively for the German Army. There are no known surviving examples of this version.

Around 1927, Chiffriermaschinen AG, the Enigma manufacturer, started the development of a series of new machines, all derived from the commercial Enigma D. For the Reichswehr (the predecessor of the Wehrmacht), they developed the Enigma I and for various other customers (both civil and military) the Enigma K family was introduced.
At the same time, development was started on an improved - mechanically more advanced - machine that was described to customers [3] as:

Glühlampen-Chiffriermaschine "ENIGMA"
mit Zählwerk
und zwangläufiger
Kupplung der Chiffrierwalzen

Translated: Lamp-Enigma with counter and coupled cipher wheels. It has a cog-wheel driven wheel-turnover mechanism that features irregular stepping, making it cryptographically stronger than the regularly stepping Enigma D. Several models were based on this new concept.

After the first prototypes in 1927, the first machine to be released in 1928 with this mechanism was the Zählwerk Enigma A28, shown in the image on the right. It was followed a few years later, in 1930, by the Enigma Z (Z30, Mark II) and finally in 1931, by the Enigma G.

The Enigma G is in fact a special version of the Zählwerk Enigma. Only a relatively small number of these machines were ever built (approx. 200).

Three versions of the Enigma G were released, but finally the Zählwerk branch of the Enigma Family Tree died off and no further models based on this principle were developed. It is thought that the Zählwerk Enigma machines were too expensive to manufacture, hence the reason for the relatively small number of machines that were built.

 More about the Zählwerk Enigma
The cosed box The Zählwerk Enigma with the top lid open Main controls: keyboard, lamp panel and cipher wheels Close-up of the coupling lever Close-up of the cipher wheels and the power switch The interior of the machine Close-up of the drum mechanism The crank stored inside the top lid

Abwehr Enigma
The machine is sometimes called the Abwehr Enigma as it was used to some degree by the German Secret Service, the Abwehr, during WWII. It should be noted however that this was not the only cipher machine used by the Abwehr, and that the machine was also used by civil and military customers in several other countries, including Hungary and The Netherlands. Furthermore, the machine was also used during WWII by the Sicherheitsdienst (SD), the German Security Service.

The Abwehr used different wiring than the other customers of this machine. In order to keep the wiring secret, they ordered blanks (i.e. unwired wheels) from the manufacturer. Furthermore, different branches of the Abwehr used differently wired wheels and it seems likely that, during the course of the war, the wiring of the Abwehr wheels was changed several times.

Some good examples of Enigma G machines are the G-312 that was used by the Abwehr, and the G-260 that was most likely used by the Sicherheitsdienst (SD). In practice, the machine was mostly called Zählwerk Enigma (Counter Enigma) or Zählwerksmaschine, after the Enigma A28 on which is was based.
Abwehr Enigma G-312
A good example of a standard version of the Enigma G (designator Ch.15a) is the machine with serial number G-312 which is on public display at the Bletchley Park museum in the UK. It is in fact a true Abwehr Enigma, as it was used by the German Seret Service, the Abwehr, during WWII.
Machines of this type are extremely rare and Bletchley Park is one of the very few places in the world where a surviving machine can be seen. Unfortunately, the G-312 was stolen from the museum in broad daylight on 1 April 2000.

Luckily, Enigma historian David Hamer had just completed a detailed description of the machine, which was quickly spread on the internet [4]. As a result, the machine could not be traded easily and was returned several months later to BBC reporter Jeremy Paxman, shown on the right [5].
BBC presenter Jeremy Paxman with the returned Enigma G-312. Photo courtesy BBC News 2000 [5].

Before that time, the machine was considered lost for several months, until the police received letters from a man who claimed to be acting on behalf of someone who had bought the machine. In the letters he demanded GBP 25,000 for safe return of the machine. Although the museum had agreed to pay the ransom, the 6 October deadline wasn't met and no money was payed [1].

Two weeks later the machine was sent to BBC reporter Jeremy Paxman, who received the parcel in his office at the London Television Centre. The machine appreared to be undamaged, but the three cipher wheels were missing. They were also returned safely at a later date.

In November 2000, the British Police arrested 50-year-old Dennis Yates, an antiques dealer from Derbyshire. He admitted sending the letters to the police and returning the machine to Jeremy Paxman, but persisted that he was acting on behalf of someone else. He refused to reveal the name of the person(s) involved, claiming that he had received death threats [1]. Somehow the situation had run out of his control. He was subsequently jailed for 10 months. Christine Large, then the director of the Bletchley Park Museum, later wrote a book based on the case [6].
Default wiring
The table below shows the default wiring of the Enigma G, which is identical to the wiring of the commercial Enigma D. The only difference is the number of notches on each wheel. The wiring and the position of the notches is identical to that of the Zählwerk Enigma A28. Please note that many (military) users later changed the wiring of the cipher wheels and that the German Abwehr even ordered wheels without wiring. In most cases the wiring of the UKW was not changed.

Wiring of the G-312
The table below shows the wiring of the G-312. Although the machine is believed to have been used by the German Abwehr, it is the only one every found with this wiring. Different wirings were used for different sections of the Abwehr, and also for different radio nets. It is also possible that some machines were rewired a number of times during their lifetime. Note the rewired UKW.

This machine is currently on public display in the museum at Bletchley Park.
Wiring of the G-260
In March 1945, just before the end of WWII, the Argentine police arrested the German spy Johann Siegfried Becker. In his posession was an Enigma model G31 with serial number G-260. Two months later, the machine was handed over to the Americans [7]. Although Becker was believed to work for the German Secret Service, the Abwehr, the G-260 was most likely used by the Sicherheidsdienst (SD), the German Security Service. The SD had its own network in Argentina which was called Red by the Allies. Their Enigma G31 machines were wired differently from the machines that the Abwehr used in Argentina (known to the allied codebreakers as Green) [11].

The current whereabouts of this machine are unknown.
The strange case of Enigma G-111   Ch.15b
In May 2009, a very rare variant of the Zählwerk Enigma model G31 turned up at auction house Hermann Historica in Munich (Germany). The machine with serial number G111 would be up for auction in October 2009. It is a very rare example of the Ch.15b variant of the Enigma G.
It appeared to be a G31 model from the first production batch (the numbers started with G101) that had a hitherto unseen connector at its left side. In July 2009, we were given the opportunity to research this machine in detail.

Crypto Museum has since written and published a paper about this rare machine, which is available from our special G-111 page.

 More information
Technical Description
The Zählwerk Enigma is clearly based on the design of the Enigma D, albeit with some additional features and improvements. Most of these features are described in the two German patents DE534947 [8] and DE579555 [9]. The most striking difference with the other Enigma models is the way in which the wheels are moved. In the earlier Enigma D (and also in the later Enigma I that was used by the German Army), the wheels are moved by means of pawls, rachets and notches. As a result, the wheel-stepping mechanism of these machines can only move forward.

In the Zählwerk Enigma, however, the wheels are moved by a cog-wheel based gearbox. The number of notches on each wheel has been increased drastically, and is different for each wheel. The longest cipher period is obtained when different prime numbers (relative to 26) are used for the number of notches on each wheel. And this is exactly what is done in this machine.

Another difference with the Enigma D is that the UKW (reflector) can not only be set to any of 26 positions, it is also moved during encipherment. The three coding wheels are mounted on a spindle, just like in most other Enigma machines, whilst the UKW is fitted permanently.

Each cipher wheel has a full cog-wheel with 52 teeth attached to its right side. On the left of the wheel is another cog wheel with the same spacing, but with a number of teeth missing. The presence of a pair of teeth is equivalent to a gap on an ordinary wheel. When the wheels are engaged, they are coupled by means of 4 small cogwheels with teeth of alternating length.

Ordinary Enigma wheels (left) and the wheels of a Zählwerk Enigma (right)

As a result, the entire mechanism can be stepped forward and backward, without losing the relation between the position of the wheels. A crank can be inserted into a hole in the body of the machine, allowing the mechanism to be wound back to the desired position. This was used to correct mistakes, but could also act as part of the crypographic key procedure.

The wheels of the standard Zählwerk machine have the same diameter as the wheels of other Enigma machines, such as the Enigma D. With the later G31 model however, smaller wheels were supplied as illustrated in the drawing below. In order to accomodate the spring-loaded contacts, they are arranged in a zig-zag pattern and the contact pads at the other side have an oval shape.

The wheels of the initial Zählwerk machine (left) and the Enigma model G31 (right)

Most machines were supplied with just 3 wheels that could be mounted on the spindle in 6 different orders (3 x 2 x 1). These wheels (I, II and III) had 17, 15 and 11 notches respectively. The positions of these notches are identical for all recovered machines, regardless of their wiring and regardless of the customer. It is known that some machines were supplied with more that three wheels. The Hungarian Army, for example had Enigma G31 machines with five wheels.

It is very likely that the Zählwerk Enigma was originally intended for commercial use, as the wiring of the UKW and the coding wheels of some recovered machines is identical to the wiring of the commercial Enigma D. In some cases, the wheels were rewired by the customer, but in many cases the wiring of the UKW was left unchanged. The Abwehr ordered unwired wheels [10].
  1. BBC, 2000: Wartime coding machine stolen
    1 April 2005. Retrieved February 2013.

  2. Crypto Museum, Photographs of Enigma G-312
    Bletchley Park, August 2002, November 2004.

  3. Chiffriermaschinen Aktiengesellschaft, Herrn Direktor Walter Edström
    Offering for Schreibende Enigma and Glühlampenmaschine mit Zählwerk (German).
    16 September 1929. Crypto Museum #300304. 1

  4. David Hamer, G-312: An Abwehr Enigma
    Cryptologia, January 2000, Volume XXIV, Number 1.

  5. BBC, Photograph of Jeremy Paxman with Enigma G-312
    Copyright BBC News, 2000. Retrieved November 2005.

  6. Christine Large, Hijacking Enigma
    31 May 2004. ISBN 978-0470863473.

  7. US Military Attaché in Argentina, Intelligence Report about G-260
    US Navy Intelligence Division. 30 May 1945. NARA CBKI 13, Box 5395, nr. 1574 1

  8. German Patent DE534947 (9 November 1928)
    Patent for the cog-wheel driven wheel-turnover mechanism and the Ringstellung.

  9. German Patent DE579555 (17 November 1928)
    Patent covering multiple notches on the wheels fixed to the index ring.

  10. Frode Weierud, Personal correspondence
    Crypto Museum, May 2009.

  11. Phil Marks, Personal correspondence
    Crypto Museum, May 2013.

  1. Documents kindly supplied by Frode Weierud [10].

Further information

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