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Enigma G
Zählwerksmaschine G31

Enigma G, or Enigma Model G31, is an Enigma cipher cipher machine, developed around 1931 by Chiffriermaschinen AG (ChiMaAG) in Berlin (Germany). It is a so-called Zählwerksmaschine (counter machine) and can be seen as a smaller version of its predecessor, Enigma Model A28. The machine is popularly known as Abwehr Enigma and also by its factory designator Ch.15a. During World War II, the codebreakers at Bletchley Park (BP) called it the 11-15-17 machine, after the number of turnover notches on the rotors. Approx. 350 units were manufactured [12].

Enigma G is one of the few Enigma machines that feature irregular stepping. It has three electrically wired cipher rotors with 17, 15 and 11 turnover notches respectively, that are driven by firmly coupled cogwheels and pinion wheels. Mistakes can be corrected by turning the rotors one or more steps backward in full synchronism, using a crank that can be inserted at the side.

In addition, the settable reflector (UKW) moves during encipherment. To the right of the rotors is a 4-digit counter which increments with every key press, so that it is easier to correct mistakes.
  

The machine is different from all other Enigma models, not only because it has a counter and a cogwheel-driven stepping mechanism, but also because it is smaller, has a sloped lamp panel and a lever protruding the top lid, just behind the rotors. Furthermore, the rotors are smaller, as a result of which their contact pins have a zig-zag arrangment. Enigma G can be seen as a further evolution of Zählwerk Enigma A28, of which the development started between 1926 and 1928.

The machine was produced in small quantities from 1931 to 1943. From surviving documents it is estimated that around 350 units were made [12]. It's main users were the Hungarian Army, the Dutch Navy, the German intelligence and security service — the Abwehr — and very few others. There were three versions of which two have meanwhile been rediscovered. The rare Enigma G reached the headlines in 2000, when the G312 was stolen from the Bletchley Park Museum. The machine shown above (G219) was used by the Royal Dutch Navy and was purchased in 1938/39.

Enigma G seen from the left
Enigma G outside the wooden transit case
G219 without wooden transit case
Enigma G seen from the side
Enigma G with open lid
Enigma G interior
Rotors and spindle - showing contact pin side
Contact pin (left) and contact plate (right) side
A
×
A
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Enigma G seen from the left
A
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Enigma G outside the wooden transit case
A
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G219 without wooden transit case
A
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Enigma G seen from the side
A
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Enigma G with open lid
A
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Enigma G interior
A
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Rotors and spindle - showing contact pin side
A
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Contact pin (left) and contact plate (right) side

Features
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 rotors, the lamp panel and the keyboard. Note the sophisticated cog­wheel driven rotor stepping gear that is an exclusive feature of Enigma A28 and Enigma G31.

Internal view of the G111

This machine does not have a large battery compartment 1 like the other models, but a much smaller one that is integrated with the power switch assembly at the top right. After releasing the two locking bolts, the switch assembly can be removed and the batteries are exposed. 2 The battery box has room for two regular (civil) 4.5V flat batteries that are connected in parallel. On one version a printer socket is available at the left side. It is shown here with a dotted line.

Differences with earlier machines
The enclosure and the wooden transit case of Enigma G31 are smaller than those of an Enigma K (A27) or Zählwerk Enigma A28. This was probably done to make it more portable. The transit case measures 25 × 27 × 16.5 cm. The following differences with the above machines have been recorded. The features in the rightmost column are shared with the Zählwerk Enigma A28.

  • Smaller transit case
  • Smaller enclosure
  • Smaller cipher wheels
  • Smaller battery compartment 1
  • Sloped lamp panel
  • No lamp test socket
  • Letters and symbols
  • Cogwheel driven stepping gear
  • Gear coupling lever
  • Crank (for corrections)
  • Counter
  • No double-stepping anomaly
  • Driven UKW
  1. Other models use a large 4.5V Wehrmacht battery. The G31 accepts two 4.5V flat batteries.
  2. Thanks to Glenn Miranker for first noticing the battery compartment in 2015.

Zählwerk Enigma A28 (left) and G31 (right)
Enigma G31 (front) and its predecessor A28 (rear)
Rotors of the A28 and the G31
Crank receptacle
Lamp panel hold-down screw
Rotors and coupling lever
Power switch, power terminals and letter counter
Coupling lever
Releasing the coupling
UKW, cipher rotors and counter
UKW and three cipher rotors
Ratchet wheel and actuator (in rest position)
Entry wheel (ETW) with driving gear and ratchet wheel
Settable and movable reflector (UKW)
Coupling pinion between two rotors
Entry wheel (ETW) with driving gear and counter
B
×
B
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Zählwerk Enigma A28 (left) and G31 (right)
B
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Enigma G31 (front) and its predecessor A28 (rear)
B
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Rotors of the A28 and the G31
B
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Crank receptacle
B
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Lamp panel hold-down screw
B
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Rotors and coupling lever
B
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Power switch, power terminals and letter counter
B
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Coupling lever
B
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Releasing the coupling
B
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UKW, cipher rotors and counter
B
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UKW and three cipher rotors
B
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Ratchet wheel and actuator (in rest position)
B
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Entry wheel (ETW) with driving gear and ratchet wheel
B
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Settable and movable reflector (UKW)
B
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Coupling pinion between two rotors
B
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Entry wheel (ETW) with driving gear and counter


Versions
Zählwerk Enigma A28 (the predecessor of the G31)
Standard version of Enigma G31
Enigma G31 with printer socket
Enigma G31 with plugboard
Ch.15 — Predecessor
Enigma G31 is based on the design of Enigma A28, which was developed in 1928. Although the two machines are interoperable, the A28 has the more usual form factor of the Enigma K (A27) and the rotors have the same dimensions as the rotors of the Enigma K and Enigma I.

Like the Enigma G, it has a driven UKW and a cogwheel-based stepping mechanism.

 More about Enigma A28

  

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 G312 that is part of the Bletchley Park collection, and the G260 that was found in Argentina towards the end of World War II. The machine described on this page, is also of this type.

 Restoration of the G219

  

Ch.15b — With printer socket
This version is functionally identical to the standard model (Ch.15a), but has a circular socket at the left, that allows another machine — such as the Enigma H — to be used as a printer attachment. A good example is Enigma G111, shown in the image on the right, and the relic of G110 that is on public display at Bletchley Park.

Ch.15b is also known as Enigma mit Zählwerk und Küpplung (Enigma with counter and link).

 More information

  

Ch.15c — With plugboard
According to the personal notes of Enigma-developer Willi Korn, this version had a fixed UKW and a plugboard [12]. However, as the standard Steckerbrett (plugboard) was used exclusively for the military machines, it is likely that an alternative design was used instead — perhaps an externally pluggable reflector.

At least two units were manufactured, but there are no known surviving examples of this version.

  





Rotors
The rotors of the Enigma model G31 are smaller than those of all other Enigma machines. 1 The largest diameter is approx. 85 mm (3.5 inches), whereas the regular rotor 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 Zählwerk Enigma A28 (left) and Enigma G31 (right)

The diagram above shows the rotors of Enigma G31 (right) next to the rotors of its predecessor Enigma A28. It is likely that the smaller rotors were developed to lower the overall weight and size of Enigma G. Like on Enigma A28, the stepping of the rotors of Enigma G31 is different from the more regular Enigma K or Enigma I. A regular Enigma rotor is driven by a ratchet wheel at its right side, 2 where­as the an Enigma G31 rotor has a cogwheel with 52 teeth in that position. In addition, the left size of a regular rotor has a ring with one or two turnover notches, whereas an Enigma G31 rotor has a cogwheel of which some teeth are missing. This is equivalent to 11, 15, and 17 notches respectively, which are responsible for the irregular stepping of the Enigma G31.

The image on the right shows an exploded view of an Enigma G rotor that was recovered from a lake in Germany in the 1980s. 3 It has been con­verted to an exploded view for demonstrations.

When inside the machine, the three rotors and the UKW are firmly coupled by means of small pinion wheels that are located behind the rotors. Once coupled, the rotors can no longer be moved manually. In order to alter the basic setting of the machine, the rotors first have to be decoupled. This is done with the coupling lever, which is located behind the rotors.
  

Most machines were supplied with just three rotors that could be installed on the spindle in six different orders (3 x 2 x 1). These rotors (I, II and III) had 17, 15 and 11 notches 4 respectively. The positions of these notches are identical on all surviving machines, regardless of their wiring and regardless of the customer. It is known that some machines were supplied with more than three wheels. For example, the Hungarian Enigma G31 machines (Ch.15b) came with five rotors, in which case three of these rotors could be picked and installed in 60 different orders (5 × 4 × 3). It is likely that the additional rotors (IV and V) had 9 and 7 notches respectively.

 Technical description

Key settings
When creating an encrypted message, the operator has to use two cryptographic keys: (1) a daily key – picked from a key list – which is changed every 24 hours, and (2) a message key – unique for each new message – which consists of 4 letters that are randomly picked by the operator. The message key is always sent at the beginning of the message, and is encrypted with the daily key.

When setting the machine up for the daily key, the operator had to open the lid of the Enigma, decouple the rotors, release the UKW and remove the spindle with the three rotors. He would then alter the Ringstellung (ring setting) for each wheel as prescribed in the key list.

This is done by pushing the small spring-loaded pin (shown in the image on the right) to the right and turning the letter ring until the desired letter is lined up with the pin. The pin is then released. Note that this is different from altering the ring setting on a rotor of a regular Enigma machine.
  

The rotors are then placed on the spindle in the prescribed order, after which the rotor stack (spindle with three rotors) is placed back in the machine beteen the ETW and the UKW. The UKW is then shifted to the right and locked in place by placing its lever in the frontmost position.

Next, the lid is closed and the four rotors (UKW and three cipher rotors) are set to the desired Grundstellung (basic setting). This is done by turning the rotors until the four letter prescribed in the key list, are visible in the four windows.

Once this is done, the coupling lever must be set to the frontmost position, so that all four rotors are firmly coupled – via the pinion wheels – to the driving cogwheel. This is done by pulling the lever upward, shifting it to the front and then releasing it. The machine is now ready for use. The user must now create the message key.
  

This is done by randomly picking four letters of the alphabet (e.g. ADXH). He then types these four letters on the machine (which is configured for the basic setting) and writes down the output (e.g. BSTU). In case of the Abwehr, the message key had to be entered twice (e.g. BSTU ADFL).

The operator then temporarily releases the coupling and sets the rotors to his randomly picked message key (ADXH), after which the coupling is engaged again. He then enters the message and writes down the output. At the receiving end, the operator sets his machine to the daily key (as per key list) and enters the first four-letter group. This reveals the message key. He then sets the rotors to the message key, enters the rest of the message and writes down the plaintext output.

Abwehr
In case of the Abwehr, the operator had to enter the first eight letters of the message, which revealed the message key twice. This was done as an extra check to ensure that the correct settings had been used. At the same time, this procedure of entering the message key twice, introduced a cryptographic weakness that was exploited by Dilly Knox and his team at Bletchley Park. Although the German Army was aware of this weakness — they abandonned this procedure at the beginning of the war — this knowledge apparently never reached the Abwehr, as for the remainder of the war they kept using the double encryped message key.

  1. With exception of the numbers-only Enigma Z30.
  2. The ratchet and pawl arrangement is responsible for the double-stepping anomaly of the middle rotor of a regular Enigma [14].
  3. At the end of the war, the Germans threw much of their equipment into deep lakes, to prevent them from falling into enemy hands.
  4. For analogy with a regular Enigma, we use the term notches to indicate a turnover position. In reality however, it is the presence of a pair of teeth on an Enigma G31 rotor that causes a turnover.

Rotor set - left angle view
Rotor set - right angle view
Rotor set - front view
Full cogwheel and notched cogwheel
Full cogwheel and notched cogwheel
Spring-loaded ring setting pin (here at 'A')
Reflector (UKW)
Driving gear
Rotors and spindle
Rotors and spindle - showing contact pin side
Rotors and spindle - showing contact pin side
Contact pin (left) and contact plate (right) side
Entry wheel (ETW) with driving gear and ratchet wheel
Settable and movable reflector (UKW)
Stepping mechanism seen from the front
Ratchet wheel and actuator (in rest position)
Releasing the UKW
Gear mechanism decoupled
UKW, cipher rotors and counter
UKW and three cipher rotors
Enigma G31 rotor I with serial number G274 - left angle view
Enigma G31 rotor I with serial number G274 - right angle view
Enigma G31 rotor I with serial number G274 - frontal view
Enigma G31 rotor I with serial number G274 - left view detail
C
×
C
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Rotor set - left angle view
C
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Rotor set - right angle view
C
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Rotor set - front view
C
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Full cogwheel and notched cogwheel
C
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Full cogwheel and notched cogwheel
C
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Spring-loaded ring setting pin (here at 'A')
C
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Reflector (UKW)
C
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Driving gear
C
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Rotors and spindle
C
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Rotors and spindle - showing contact pin side
C
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Rotors and spindle - showing contact pin side
C
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Contact pin (left) and contact plate (right) side
C
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Entry wheel (ETW) with driving gear and ratchet wheel
C
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Settable and movable reflector (UKW)
C
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Stepping mechanism seen from the front
C
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Ratchet wheel and actuator (in rest position)
C
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Releasing the UKW
C
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Gear mechanism decoupled
C
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UKW, cipher rotors and counter
C
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UKW and three cipher rotors
C
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Enigma G31 rotor I with serial number G274 - left angle view
C
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Enigma G31 rotor I with serial number G274 - right angle view
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Enigma G31 rotor I with serial number G274 - frontal view
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Enigma G31 rotor I with serial number G274 - left view detail


History
Around 1926, the Enigma manufacturer — Chiffriermaschinen AG — started the development of a series of new machines, all based on the chassis of commercial Enigma D (A26). For the Reichs­wehr (the predecessor of the Wehrmacht), they developed the Enigma I and for various other civil and military customers the Enigma A27 (later also known as Enigma K) was introduced. At the same time, development was started of an improved machine that was described as follows: [3]

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

Translated: Lamp-Enigma with counter and coupled cipher rotors. It has a cogwheel-driven rotor-turnover mechanism that features irregular stepping, making it cryptographically stronger than the regularly stepping Enigma D and K. Several models were based on this new concept. All these machines have a counter that registers the number of key-presses, which is why they are also known as Zählwerksmachine (counter machine) or Zählwerk Enigma (counter Enigma).

After the first prototypes in 1927, the first machine with this mechanism was released in 1928 as the Zählwerk Enigma A28. Two versions of this machine exist (Mk.1 and Mk.2), the latter of which is shown in he image on the right. It has the same form factor as a regular Enigma K.

A few years later, around 1930, a numbers-only version of the machine was released as the Z30.

Enigma A28 eventually evolved into the smaller Enigma G (or model G31), which was released in 1931. The two Enigma models are interoperable.
  

Three versions of the Enigma G were developed, but finally the Zählwerk branch of the Enigma Family Tree died off around 1943 and no further models based on this principle were developed. It seems likely that the Zählwerk Enigma machines were too expensive to manufacture, hence the the relatively small number of approximately 350 Enigma G machines that were manufactured.

 More about Zählwerk Enigma A28
 Full history of the Enigma G, by David Kenyon and Frode Weierud




Customers
Hungarian Army
Assuming that the serial numbers of the Enigma G started at G101, the Hungarian Army was the first customer the machine. In 1931 they purchased 24 machines numbered G101 to G124, all of which were designated Ch.15b and had a special socket for the connection of an external printing device — in practice an Enigma H29 — towards the rear of its left side [12][13].

Surviving examples of the Hungarian machines are G110, a relic which is on public display at the Bletchley Park Museum [12], and G111, which is owned by a private collector in Germany. The latter — G111 — was described in great detail in 2009 by Paul Reuvers and Marc Simons in the paper Enigma G111: A rare version of Zählwerk Enigma G31 [13]. It has since been restored.

 More information about the G111
 Our paper about the G111

Royal Dutch Navy
Another early customer of the Enigma G was the Royal Dutch Navy. They had already purchased six Enigma model A28 machines – the predecessor of Enigma G – in 1928 and ordered their first Enigma G (G128) in 1931, shortly after the Hungarians. It was probaly used for evaluation and for testing the compatibility with the Enigma A28 machines. They bought at least another 71 Enigma G machines between 1937 and 1939, all of which were used aboard naval ships and submarines.

Another six machines were ordered just before the outbreak of WWII, and were delivered on 9 May 1940, one day before the German invasion of The Netherlands [12]. It is believed that these machines were subsequently confiscated by the Germans along with an unknown number of other Enigma G machines and rotors that were kept in storage at the (bombed) Frederik Kazerne in The Hague. At least one of the Dutch machines, G209, was rewired in 1943 and used by the Abwehr in Argentina [12]. It is clear that the Dutch Navy was the largest customer of Enigma G machines outside Germany. They purchased at least 93 Counter Enigma machines (A28 and G31) [12].

A good example of an Enigma G machine used by the Dutch Navy, is the G219 in the collection of Crypto Museum. Similar machines are held in the collections of the Dutch Naval Museum in Den Helder (Netherlands) and in the internal collection of the Dutch intelligence service AIVD.

 Detailed description of the G219

Abwehr
The largest Enigma G customer was the German intelligence and security service — the Abwehr — which is why the machine is often dubbed Abwehr Enigma. This name is a disnomer however, as it was not the only cipher system that was used by the Abwehr, nor was the Abwehr the only user of the Enigma G31. The machine had various other customers in Germany and abroad.

The Abwehr used differently wired wheels and sometimes also a differently wired reflector (UKW). To hide the wiring even from the manufacturer, they were ordered as blanks (i.e. unwired) and were wired by the Abwehr themselves. Furthermore, different Abwehr radio nets used differenly wired wheels, and it seems likely that during the course of the war, some of them were rewired.

Good examples of Enigma G machines that were used by the Abwehr, are G312 in the collection of the Bletchley Park Museum, and G260 that was used by the Abwehr or the Sicherheitsdienst (SD) in Argentina. From 1944 onwards, the Abwehr and the SD both resided under the RSHA.

 More about the Abwehr




Cryptanalysis
During World War II (WWII), the Abwehr Enigma signals were first broken at Bletchley Park in October 1941 by codebreaker Dilly Knox. It was achieved by studying the double encrypted message indicators in combination with a technique known as rodding. Rodding was an improvement of a codebreaking technique developed in 1927 by Hugh Foss at GC&CS, after investigating a commercial Enigma D that had been purchased by Knox in Vienna in 1928 [15].

The break of the Abwehr Enigma led to the establishment of Intelligence Service Knox (ISK) that was located in the Cottage at Bletchley Park.

At the Cottage, Knox and 'his girls' worked on solving non-steckered Enigma, which included the Enigma K used by the Italian Navy and the Enigma G used by the Abwehr. By the end of the war, the people at ISK had decrypted and disseminated no less than 140,800 messages.

The Cottage is also the place where codebreaker Alan Turing later worked on Naval Enigma M4.
  



Special cases
Below is an overview of the Enigma G machines we've encountered over the years. As each machine has a fascinating history, further information can be found by clicking the images.

G111 - Printer socket   Ch.15b
Enigma G111 — also known by its factory designator Ch.15b — belongs to the very first batch of Enigma G machines. It was supplied to the Hungarian Army and features a 28-contact connector at its left side, used for connection of an external printing device (i.e. Enigma H29).

In 2009, the G111 turned up at German auction house Hermann Historica in Munich (Germany) and was subsequently investigated by Crypto Museum. It is described here in great detail.

 More information

  

G219 - Royal Dutch Navy
Enigma G219 was delivered to the Royal Dutch Navy in 1938 or 1939 and was used during WWII by the Dutch Navy operating in British waters.

After the war it was used on communication links between The Netherlands, the Dutch Antilles and the Dutch East Indies. The G219 is now part of the Crypto Museum Collection. Its full history and restoration is described here.

 More information

  
Enigma G interior

G312 - The stolen Enigma
Enigma G312 was used by the German Abwehr and is now part of the internal collection of GCHQ. It is on permanent public display at the Bletchley Park Museum (UK).

This machine attracted a lot of attention after it was stolen from the museum on 1 April 2000. Luckily it was returned to the museum later that year, after it had been sent anonymously to BBC presenter Jeremy Paxman.

 More information

  



Wiring
Standard wiring
The table below shows the default wiring of the Enigma G, which is identical to the wiring of the commercial Enigma K. 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 some (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.

Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover #
ETW QWERTZUIOASDFGHJKPYXCVBNML      
I LPGSZMHAEOQKVXRFYBUTNICJDW ACDEHIJKMNOQSTWXY SUVWZABCEFGIKLOPQ 17
II SLVGBTFXJQOHEWIRZYAMKPCNDU ABDGHIKLNOPSUVY STVYZACDFGHKMNQ 15
III CJGDPSHKTURAWZXFMYNQOBVLIE CEFIMNPSUVZ UWXAEFHKMNR 11
UKW IMETCGFRAYSQBZXWLHKDVUPOJN      

Wiring of the G312
The table below shows the wiring of the G312. 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 possible that some machines were rewired a number of times during their lifetime. Note the rewired UKW.

Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover #
ETW QWERTZUIOASDFGHJKPYXCVBNML      
I DMTWSILRUYQNKFEJCAZBPGXOHV ACDEHIJKMNOQSTWXY SUVWZABCEFGIKLOPQ 17
II HQZGPJTMOBLNCIFDYAWVEUSRKX ABDGHIKLNOPSUVY STVYZACDFGHKMNQ 15
III UQNTLSZFMREHDPXKIBVYGJCWOA CEFIMNPSUVZ UWXAEFHKMNR 11
UKW RULQMZJSYGOCETKWDAHNBXPVIF      

This machine is currently on public display at the Bletchley Park Museum.


Wiring of the G260
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 G260. 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 G260 was most likely used by the Sicherheidsdienst (SD), the German Security Service. The SD had its own network in Argentina which was called Rot (red). Their Enigma G31 machines were wired differently from the machines used by the Abwehr in Argentina (known to the allied codebreakers as Green) [11].

Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover #
ETW QWERTZUIOASDFGHJKPYXCVBNML      
I RCSPBLKQAUMHWYTIFZVGOJNEXD ACDEHIJKMNOQSTWXY SUVWZABCEFGIKLOPQ 17
II WCMIBVPJXAROSGNDLZKEYHUFQT ABDGHIKLNOPSUVY STVYZACDFGHKMNQ 15
III FVDHZELSQMAXOKYIWPGCBUJTNR CEFIMNPSUVZ UWXAEFHKMNR 11
UKW IMETCGFRAYSQBZXWLHKDVUPOJN      

The current whereabouts of this machine are unknown.


Wiring of the G111
The table below shows the wiring of the G111 (version Ch.15b), that was from the first batch of Enigma G machines ever manufactured. It was part of a batch of 24 machines (G-101 to G-124) that were delivered to Hungary, probably in 1931. It is believed that this machine was used by the Hungarian Army or the Hungarian intelligence services, who bought it alongside the Enigma H29. Note that only wheels I, II and V were found with this machine.  More about the G111

Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover #
ETW 1 QWERTZUIOASDFGHJKPYXCVBNML      
I WLRHBQUNDKJCZSEXOTMAGYFPVI ACDEHIJKMNOQSTWXY SUVWZABCEFGIKLOPQ 17
II TFJQAZWMHLCUIXRDYGOEVBNSKP ABDGHIKLNOPSUVY STVYZACDFGHKMNQ 15
III ? CEFIMNPSUVZ UWXAEFHKMNR 11
IV ? ? ? ?
V QTPIXWVDFRMUSLJOHCANEZKYBG AEHNPUY SWZFHMQ 7
UKW 2 IMETCGFRAYSQBZXWLHKDVUPOJN      

As we can learn from the above table, the number of notches as well as the turnover positions of wheels I and II are identical to those on the same wheels of other Zählwerk machines (17 and 15 notches respectively). It is most likely that the (unknown) wheel III had 11 notches and that the turnover positions are the same as on other machines. They are shown here in red. It is likely (but not certain) that the number of turnover notches on the unknown wheel IV is 9. The current where­abouts of this machine are unknown.

 More about Enigma G111

  1. This machine has the standard wiring of the ETW for a commercial machine.
  2. The UKW is also wired in the standard fashion for a commercial machine.

Technical Description
Zählwerk Enigma (counter Enigma) is based on the design of the Enigma D (A26) and Enigma K (A27), but has additional features and improvements. Most of these extras are described in two German patents: DE534947 [8] and DE579555 [9]. The most important difference with earlier models is the way in which the rotors advance when a key is pressed.

In the earlier Enigma K, and also in the later Enigma I, the rotors are advanced by means of pawls, ratchets and notches. As a result, a rotor can only step forward. Under certain conditions, the middle rotor can make an additional step on two successive key presses. This is known as the double stepping anomaly [14]. This stepping behaviour is also known as Enigma stepping.


With Zählwerk Enigma however, the rotors are advanced by a cogwheel-driven mechanism that does not suffer from the double stepping anomaly. In addition, the number of notches on each wheel has been increased. It is different for each wheel, does not share a common factor and cannot divide 26. By using 17, 15 and 11 notches respectively, the longest possible cipher period is obtained. This stepping behaviour is also known as irregular stepping.

Another difference with Enigma K is that the reflector (UKW) can not only be set to any of 26 positions, but is also moved during encipherment. This is known as a movable or driven UKW. The three cipher rotors are mounted on a removable spindle, just like with other glowlamp Enigma machines, but the movable UKW is fitted permanently. It cannot be removed easily.

Each rotor has a full cogwheel with 52 teeth attached to its right side. On the left side of the rotor is another cogwheel with the same spacing, but with a number of teeth missing. The presence of a pair of teeth is equivalent to a notch on an regular Enigma rotor. When the rotors are engaged, they are firmly coupled by means of 4 small pinion wheels with teeth of alternating length.

Both sides of an Enigma G rotor

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 theoretically also be used as part of the cryptographic procedure.

Regular Enigma wheels (left) and the wheels of Zählwerk Enigma A28 (right)

The rotors of the initial Zählwerk machine — Enigma A28 — have the same diameter as the rotors of a regular Enigma, such as Enigma K. With the later G31 model however, smaller wheels are used, as illustrated below. In order to accomodate the spring-loaded contacts, they are arranged in a zig-zag pattern, whilst the contact pads at the other side have an oval or tear-drop shape.

The wheels of the Zählwerk Enigma model A28 (left) and the Enigma model G31 (right)

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

It is very likely that the Zählwerk Enigma was originally intended for commercial use, as the wiring of the UKW, and in some cases the wiring of the rotors as well, was identical to the wiring of the commerial machines. Sometimes the rotors were rewired by the customer, but in most cases the wiring of the UKW was left unchanged. The Abwehr ordered unwired rotors [10].

  1. For analogy with a regular Enigma, we use the term notches to indicate a turnover position. In reality however, it is the presence of a pair of teeth on an Enigma G31 rotor that causes a turnover.




Interior
The Enigma G is usually mounted inside a wooden transit case with a hinged lid. It is fixated to the bottom of the case by means of four large bolts. After removing these bolts, the machine can be lifted from the case. At the bottom of the chassis are 4 rubber pads that may have dried out.

The machine itself is housed in a folded metal enclosure with a hinged lid, painted with black wrinkle paint. The interior can be accessed by removing the U-shaped case shell, which is held in place by 4 screws at the left, 4 at the right and 8 at the rear. After removing all 16 screws, the case shell and the hinged lid can be taken off.

A separate panel at the front – held in place by 6 screws – should also be removed in order to access the keyboard. The image on the right shows the machine after the case shell, the top lid and the front cover have been removed.
  

Note that the battery compartment of the machine shown above (G219) is currently missing. It should normally be present in the rear right corner. At the front is the keyboard with its 26 keys protruding a staircase-shaped panel. Each key actuates a black bar at the bottom – which causes the rotors to step – and activates a single-pole single-throw (SPST) switch mounted below it.

Behind the keyboard is the lamp panel with its 26 flat-faced lightbulbs. Unlike in other Enigma machines where the lamp panel is mounted horizontally, it is mounted under a 30° angle, so that the lamps lign up with the sloped top lid.

Behind the lamps is the stepping mechanism with the cipher rotors. It consists (from right to left) of an entry disc (ETW), three cipher rotors (I, II and III) and a movable reflector (UKW). The image on the right shows the rotor stepping mechanism after the keyboard and lamp panel have been removed temporarily for repair.
  

In the Enigma G, the rotors are firmly coupled by means of small pinion wheels located at the the rear of the rotors. When altering the position of the rotors (Grundstellung), the pinions must first be disengaged by placing the coupling (i.e. the large lever at the top) in the rearmost position.

Once the mechanism is decoupled and the UKW is released, the spindle with the rotors can be removed. This leaves us with an empty stepping mechanism in which the pinions and the sping-loaded rubber guide wheels are clearly visible.

At the far right is the entry disc (ETW), which is firmly fitted to the chassis. Around the ETW are a ratchet wheel and a cogwheel with 52 teeth. When pressing a key, an actuator advances the ratchet wheel by one step. At the same time, the cogwheel advances by two teeth. The image on the right shows the actuator in its rest position.
  

Note that the ratchet wheel and the cogwheel are part of a cylinder that revolves around the ETW, whilst the ETW itself stays in place. The actuator and its adjustment are further described here. The large cogwheel mates with a pinion wheel at the rear, which in turn drives the rightmost rotor. It also drives the 4-digit counter that is fitted at the far right of the chassis. For further details about the machine's interior, please refer to our page about the restoration of the G219.

 Restoration of the G219

Enigma G seen from the side
Rear view
Enigma G rotor unit
Rotor unit front view
Stepping mechanism seen from the front
Ratchet wheel and actuator (in rest position)
Coupling pinion between two rotors
Releasing the UKW
Enigma G interior (battery compartment removed)
Enigma G interior seen from the rear right
Interior seen from the rear left
Interior seen from the front left
Interior - seen from the front
Interior - seen from the rear
Interior - seen from the right
Interior - seen from the left
Keyboard switches
chassis bottom panel with keyboard-actuated stepping seesaw
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Enigma G seen from the side
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Rear view
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Enigma G rotor unit
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Rotor unit front view
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Stepping mechanism seen from the front
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Ratchet wheel and actuator (in rest position)
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Coupling pinion between two rotors
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Releasing the UKW
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Enigma G interior (battery compartment removed)
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Enigma G interior seen from the rear right
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Interior seen from the rear left
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Interior seen from the front left
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Interior - seen from the front
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Interior - seen from the rear
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Interior - seen from the right
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Interior - seen from the left
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Keyboard switches
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chassis bottom panel with keyboard-actuated stepping seesaw

Specifications
  • Device
    Rotor cipher machine
  • Brand
    Enigma
  • Type
    G
  • Model
    G31
  • Designator
    Ch.15a, Ch.15b or Ch.15c
  • Predecessor
    A28 (Ch.15)
  • Successor
    none
  • Manufacturer
    ChiMaAG, (later: H&R) 1
  • Country
    Germany
  • Years
    1931-1943 (est.)
  • Quantity
    350 (est.)
  • Customers
    see below
  • Rotors
    3 (optionally choosen from a set of 5)
  • Turnovers
    17, 15, 11, ?, 7
  • Reflector
    Settable and movable (driven)
  • Wiring
    see above
  • Stepping
    Irregular (cogwheel gear)
  • Plugboard
    No 2
  • Extras
    Crank, green filter (optional), maintenance booklet
  • Dimensions
    270 × 250 × 165 mm
  • Weight
    9.4 kg (7.2 kg without the wooden case)
  • Quantity
    350 (est.)
  1. From 1923 to 1934, the name of the company was Chiffriermaschinen AG (ChiMaAG). It was succeeded in 1934 by Heimsoeth und Rinke (H&R).
  2. Only on the Ch.15c version, which was probably not taken into production.

Customers
Nomenclature
The Enigma G is known under the following names:

  • Enigma G
  • Enigma Model G31
  • Abwehr Enigma
  • Ch.15a
  • 11-15-17 machine
  • Zählwerk Enigma G31
  • Zählwerksmaschine G31
  • Counter Enigma
Known serial numbers
Below is a list of known Enigma G serial numbers, mainly based on surviving TICOM documents, complemented by our own findings. Detailed information and backgrounds can be found in the article Enigma G: The Counter Enigma by David Kenyon and Frode Weierud of May 2019 [12].

  • G101 - G124
    1931
    Hungarian Army 1
  • G128
    1931
    Dutch Navy
  • G138 - G141
    1938 (est.)
    Dutch Navy 2
  • G145 - G188
    1938
    Dutch Navy
  • G189 - G194
    1938
    Abwehr 6
  • G195 - G221
    1939 (est.)
    Dutch Navy
  • G209
    ?
    Abwehr (Rot) 4,5
  • G222 - G224
    ?
    I.G. Farben
  • G227 - G228
    1939 (est.)
    Abwehr (Grün) 4
  • G247
    ?
    OKW/Chi
  • G249 - G250
    ?
    OKW/In 7/IV
  • G256 - G257
    ?
    I.G. Farben
  • G260
    ?
    Abwehr (Rot) 4
  • G263
    ?
    Abwehr (Rot) 4
  • G275
    ?
    OKW/Chi
  • G292
    ?
    DWStK
  • G298 - G303
    1940
    Dutch Navy 3
  • G312 - G317
    ?
    Abwehr 6
  • G404 - G407
    ?
    I.G. Farben
  • G426
    ?
    ?
  1. All these were modified (Ch.15b) for connection to printing Enigma H29.
  2. Neither the delivery date nor the exact quantity is known [12].
  3. These machines were delivered on 9 May 1940, one day before the German invasion of The Netherlands. It is likely that these machines were confiscated by the Germans [12].
  4. Used by the Abwehr in Argentina.
  5. G209 had initially been sold to the Dutch Navy and was confiscated by the Germans when they invaded The Netherlands in May 1940. It was rewired for the Abwehr in August 1943 [12].
  6. Uncertain, but likely.

Surviving machines
Surviving rotors
  1. Thanks to Günter Hütter for creating the exploded view display [16].

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

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

  3. David Kenyon and Frode Weierud, Enigma G: The Counter Enigma
    Bletchley Park and Crypto Cellar Research, 5 May 2019. Updated 15 June 2019.

  4. Frank Carter, The Abwehr Enigma Machine
    Explanation of the terms 'crab' and 'lobster'.
    Undated, but probably 2017.

  5. Paul Reuvers and Marc Simons, G-111: A rare Zählwerk Enigma variant
    Full details of the Enigma G-111 (Ch.15b) with high-resolution drawings.
    Crypto Museum, August 2009. Version 1.10, 1 May 2013.  Publication  More
References
  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 G260
    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.

  12. David Kenyon and Frode Weierud, Enigma G: The Counter Enigma
    Bletchley Park and Crypto Cellar Research, 5 May 2019. Updated 15 June 2019.

  13. Paul Reuvers and Marc Simons, G-111: A rare Zählwerk Enigma variant
    Full details of the Enigma G-111 (Ch.15b) with high-resolution drawings.
    Crypto Museum, August 2009. Version 1.10, 1 May 2013.  Publication  More

  14. David Hamer: Actions involved in the 'double stepping' of the middle rotor 2
    Cryptologia, January 1997, Volume XX, Number 1.

  15. Frank Carter, The Abwehr Enigma Machine
    Explanation of the terms 'crab' and 'lobster'.
    Undated, but probably 2017.

  16. Günter Hütter, Conversion of rotor G274/I for exploded view display
    Austria, 22 October 2023.
  1. Documents kindly supplied by Frode Weierud [10].
  2. Reproduced here by kind permission from the author.

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