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Enigma D
Commercial Enigma A26 · 1926

Enigma D is an electro­mechanical rotor-based cipher machine, developed in 1926 by Chiffrier­maschinen Aktiengesellschaft (ChiMaAG) in Berlin (Germany), as the successor to the Enigma C. Like its predecessor, it uses light bulbs for its output, and is therefore known as a Glüh­lampen­chiffrier­maschine (glow lamp cipher machine). The machine is also known by its model number A26, and by the internal designator Ch. 8. It was succeeded a year later by the very similar Enigma K (A27). Enigma D was the core design on which all later Enigma models were based.

The machine has several improvements over the Enigma C. The top lid of the machine was made more accessible by adding a hinge at the rear and two hold-down bolts at the front. This made it easier to alter the basic settings. The three rotors are now mounted on a removable spindle, so that their order can be changed as well. Furthermore, the reflector (UKW) is now settable, which means that it can be set to any of 26 positions. These improvements increased the maximum number of possible settings.

On this machine, the lid is protruded by four rotors of which the current settings are visible through four windows. This is the reason why the machine is sometimes classed as a four-rotor Enigma machine. Although technically there are four rotors, the leftmost one is the UKW. It is therefore more appropriate to define it as a three-rotor machine with a settable UKW.

The machine is built on an improved die-cast chassis and the keys and lamps are arranged in the order of a standard German typewriter (QWERTZ...) rather than the alphabet (ABC...).

It seems likely that in 1926, the Enigma D was the core design on which all future designs would be (partly) based. The Enigma D itself was short-lived and was succeeded a year later – in 1927 – by the Enigma K (A27). Enigma I and Zählwerk Enigma A28 are both based on Enigma D and K.

The machine shown above has serial number A320 and was bought by GCHQ in 1926 or 1927. It was subsequently analysed by Hugh Foss, who wrote a detailed description and devised a method for breaking it [6]. Ten years later, in April 1937, it helped William Bodsworth when first breaking the Enigma traffic between the Spanish Navy (Franco) and the Italian Navy (Mussolini) [7].

Enigma D with S/N A320 [4]
Enigma D with S/N 325, taken from the first page of the user manual [2]
Closeup of a rotor inside the Enigma D with serial number A320. Note the absence of horizontal screws to hold the notch ring in place [2][3].
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Enigma D with S/N A320 [4]
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Enigma D with S/N 325, taken from the first page of the user manual [2]
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Closeup of a rotor inside the Enigma D with serial number A320. Note the absence of horizontal screws to hold the notch ring in place [2][3].

In an earlier version of this page we showed images of a commercial Enigma with serial number A818, which was actually an early Enigma K (A27) rather than an Enigma D (A26).

Differences with Enigma C
Compared to its predecessor — Enigma C — the following differences can be observed:

  • Improved die-cast chassis
  • Letters on the rotors
  • Keyboard in QWERTZ order
  • Key caps: black background, white letters
  • Settable UKW (26 positions)
  • Hinged top lid
  • Oval or circular windows in top lid
  • Spare lightbulbs in case lid
  • Removable rotor stack
Descendants of the Enigma D
In 1927, one year after the introduction of the Enigma D, several developments of improved machines were started. This led to a range of commercial and mulitary machines with improved and/or additional features. The following machines are based on the design of Enigma D.

Reichwehr D (Ch.11a), later evolved into Enigma I (Ch.11f)
Commercial Enigma K, model A27 (Ch.11b)
Counter Enigma A28 (Ch.15), the predecessor of Enigma G (G31, Ch.15a)
Numbers-only Enigma model Z30 (Ch.16)
  • Reichswehr D (Ch. 11a)
    Machine with a single-ended Steckerbrett. This eventually led to the developement of the Enigma I (Ch.11f), which was used by the Reichswehr (later: Wehrmacht) throughout WWII.

  • Enigma K (A27, Ch. 11b)
    From 1927 onwards, right up to 1944, this was the main commercial (non-Stecker) machine. Many improvements were made and many variants exist. This machine was also the base for the Enigma T (Tirpitz), the Swiss K variant and the Enigma KD.

  • Zählwerk Enigma (A28, Ch. 15)
    This was the first of a range of Enigma machines that had a counter and a cog-wheel based stepping mechanism. In addition, the rotors had multiple turnover notches. The later Enigma G machines (G31) was also based on this machine.

  • Enigma Z (Z30, Ch. 16)
    This was a numbers-only version of the Enigma machine. It had just 10 keys (0-9), 10 lamps and the rotors each had 10 contact points at either side.
Enigma D had three cipher rotors and a settable reflector (UKW). All four rotors protrude the top lid of the machine. As the UKW has a thumbwheel for setting its position, it is nearly identical to a cipher rotor. As a result, the machine is sometimes erroneously classed as a 4-rotor Enigma, but is actually a 3-rotor Enigma with a settable UKW. The UKW does not move during encipherment.

The rotors are made from steel and aluminium, and have a Bakelite inner core with 26 contact pads at one side and 26 spring-loaded pins at the other. Each rotor can be set to 26 positions, each of which is identified with a letter (A-Z).

As with earlier Enigma machines, the notch ring is attached to the body of the rotor (rather than to the letter ring). The image on the right was taken from page 113 of the book Decrypted Secrets - Methods and Maxims of Cryptology by F.L. Bauer [5]. It shows the left side of a rotor of an Enigma D, of which the notch ring is attached to the rotor body. There are no recessed screws to hold the notch ring in place [3]. As a result, the notch ring serves no cryptographic purpose.

This is fixed in later machines like Enigma K and Enigma I, in which the notch ring is attached to the letter ring and, hence, to the rotor wiring.

If we take a closer look at the photograph at the top of this page, we'll see that there are no horizontal screws to attach the notch ring to the letter ring. This means that the notch ring is instead attached to the rotor body, and that this machine is indeed a genuine Enigma D.

Rotor wiring
Standard wiring
Below is the wiring of the Enigma D and its rotors, as described in Hugh Foss' Reciprocal Enigma from 1927 [6]. The wiring is identical to the wiring of the later Enigma K (A27) and can therefore be classed as 'commercial wiring'. Note that on the rotors of the Enigma D, the turnover notch is attached to the rotor body, rather than to the alphabet ring as on Enigma K. This means that when the ring setting is changed, the turnover of the adjacent rotor will happen at a different letter. Furthermore, the position of the notch was identical on all three rotors (I, II and III).

The table below shows the position of the notch when the ring is set to 'A'. When the ring is set to 'B', the turnover will be at 'A' and so on. This means that:
Turnover = RingSetting - 1

  1. When ring is set to 'A'.

 More about rotor wiring

Operating instructions
With the first Enigma machines, the operating instructions were no more than a few A4 pages created on a typewriter. With the arrival of Enigma D, and the later Enigma K, a professionally printed instruction manual was included, complete with photographs and a fold-out at the rear.

Original operating instructions are extremely rare, but it is known that the contents of the booklet were revised several times, probably to reflect the differences between Enigma D and K.

The image on the right shows an original A5-size instruction booklet that was probably supplied with an Enigma K. The first page of this booklet however, shows a photograph of an earlier model, which is most likely an Enigma D. Although it is a low-resultion photograph, it is clear that there are no numbers printed above the upper row of lamps on the lamp panel.

The photographs in the fold-out at the rear of this booklet, show a different machine on which the numbers (0-9) are printed above the upper row of letters on the lamp panel. Likewise, the upper row of keys holds both letters and numbers. This is thought to be a property of Enigma K. Furthermore, the power selector is of a later construction, with which the knob was part of the lid. On Enigma D, the knob is part of the power selector and protrudes a cut-out in the top lid.

 Read the booklet

About the booklet

The front cover holds a stamp with the number 33, which is probably the year in which the booklet was issued (1933). In the upper right corner, the number A833 is written with a red pencil. This is probably the serial number of the machine it was issued with. As the machine with serial number A818 in the Crypto Museum Collection has been identified as an Enigma K, it is likely that A833 was also an Enigma K.
Cover of the Enigma instruction booklet
The somewhat embossed title
Page one of the booklet
Page one of the instruction booklet
Close-up of the Enigma machine shown on the first page
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Cover of the Enigma instruction booklet
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The somewhat embossed title
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Page one of the booklet
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Page one of the instruction booklet
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Close-up of the Enigma machine shown on the first page

Enigma A320
On 14 September 2021, GCHQ issued a Tweet about the visit of veteran codebraker Charlotte 'Betty' Webb to their offices at Cheltenham [7]. One of the photographs shows Betty Webb in front of a display with two Enigma machines. A text card in between them provides some details about the A320 and Hugh Foss' analysis of it [6]. Thanks to Anastasios Pingios for the transcription.

It is not clear how GC&CS heard about Enigma, the new commercial encryption device which was being sold in Germany, but the Deputy Head of GC&CS visited a friend of his who was Naval Attaché, and bought one, serial number A-320. This was passed to the cryptanalyst Hugh Foss who produced a diagnosis entitled "The Reciprocal Enigma" which listed the conditions in which material encrypted with this Enigma machine might be broken.

As it became clearer in the late 1920s and early 1930s that the German military were aiming to make Enigma their main encryption device, GC&CS cryptanalysts began to use A-320 to design attacks against traffic encrypted on Enigma machines. At this stage no operational traffic was available to them but that changed in 1936.

After the failure of General Franco's attempted coup in July 1936, Hitler sent the Condor Legion, equipped with Enigma machines, to fight for Franco in the Civil War. Two of the machines were retained at his civil and military Headquarters, while the rest were sent in conditions of maximum security to the principal military units.

The traffic passing between these units, and later, between them and the German and Italian volunteers who were sent to support Franco, was intercepted in the UK, Spanish traffic was first broken in April 1937 by cryptanalyst William Bodsworth, the first broken message being between Franco's Navy and Mussolini's Navy.

Although much further work, and cooperation with Poland and France, would be needed to break into German military use of Enigma, it was the success against the network in Spain which gave the GC&CS cryptanalysts the confidence that success was possible.

  • Device
    Rotor cipher Machine
  • Brand
  • Type
  • Model
  • Designator
  • Predecessor
    Enigma C
  • Successors
    Enigma K, Enigma I, Enigma A28, Enigma Z30
  • Manufacturer
  • Country
  • Customers
  • Rotors
    3 (removable)
  • Contacts
  • Turnovers
    1 per rotor
  • Reflector
    Settable (26 positions)
  • Wiring
    see above
  • Stepping
    Regular (Enigma stepping)
  • Plugboard
  • Extras
    Green filter
  • Dimensions
    290 × 280 × 155 mm (incl. wooden case)
  • Weight
    2.4 kg (incl. wooden case)
Known serial numbers
Surviving machines
  1. Enigma K instruction manual (German) 1
    ChiMaAG, 1933. Issued with Enigma A833.
  1. There are different versions of this booklet. The machine shown on the first page is probably an Enigma D (A26), whilst the machine shown in the fold-out at the back, is clearly an Enigma K (A27).

  1. Frode Weierud, Personal correspondence
    Enigma serial number research. Forthcoming publication. September 2022.

  2. Chiffriermaschinen AG, Enigma booklet
    Instruction booklet with images.

  3. Jakub S., Personal correspondence
    September 2022.

  4. Kelsey Griffin, Image of open Enigma A320
    Bletchley Park, Security Conference, 2010.

  5. F.L. Bauer, Decrypted Secrets - Methods and Maxims of Cryptology
    ISBN 978-3540668718. 1 January 2000 (2nd edition). p. 113.

  6. Hugh Foss, The Reciprocal Enigma 1
    TNA, HW25/14. Undated, but probably 1927/28.

  7. GCHQ, Signage near display case in internal museum
    Brief account of the purchase of A320 and Hugh Foss' analysis.
    Twitter, 14 September 2021.

  8. Anastasios Pingios, Transcription of the A320 description [7]
    15 September 2021.
  1. Document kindly provided by Frode Weierud.

Further information
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© Crypto Museum. Created: Monday 14 September 2009. Last changed: Friday, 07 June 2024 - 06:42 CET.
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