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 .
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) .
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
- 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 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.
- 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
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 cogwheel-based stepping mechanism. In addition, the wheels had multiple
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 wheels 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 sometime 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 . 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 .
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.
Below is the wiring of the Enigma D and its rotors, as described in Hugh Foss'
Reciprocal Enigma from 1927 .
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 wheel 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
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.
The booklet shown above is
available for download below [A].
At the front cover it holds a stamp with the number 33, which is
probably the year in which it 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.
On 14 September 2021, GCHQ issued a Tweet about the visit of veteran
codebraker Charlotte 'Betty' Webb to their offices at Cheltenham .
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 .
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
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.
DeviceRotor cipher Machine
SuccessorsEnigma K, Enigma I, Enigma A28, Enigma Z30
Turnovers1 per wheel
ReflectorSettable (26 positions)
SteppingRegular (Enigma stepping)
Dimensions290 × 280 × 155 mm (incl. wooden case)
Weight2.4 kg (incl. wooden case)
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).
- Frode Weierud, Personal correspondence
Enigma serial number research. Forthcoming publication.
- Chiffriermaschinen AG, Enigma booklet
Instruction booklet with images.
- Jakub S., Personal correspondence
- Kelsey Griffin, Image of open Enigma A320
Bletchley Park, Security Conference, 2010.
- F.L. Bauer, Decrypted Secrets - Methods and Maxims of Cryptology
ISBN 978-3540668718. 1 January 2000 (2nd edition). p. 113.
- Hugh Foss, The Reciprocal Enigma 1
TNA, HW25/14. Undated, but probably 1927/28.
- GCHQ, Signage near display case in internal museum
Brief account of the purchase of A320 and
Hugh Foss' analysis.
Twitter, 14 September 2021.
Document kindly provided by Frode Weierud.
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© Crypto Museum. Created: Monday 14 September 2009. Last changed: Friday, 14 October 2022 - 22:16 CET.