Enigma B →
Glow lamp Enigma machine · 1924
Enigma A, also known as Die kleine Militärmaschine (the small military
machine), was an
electromechanical rotor-based cipher machine,
introduced in 1924 by Chiffriermaschinen Aktiengesellschaft (ChiMaAG)
in Berlin (Germany).
It was the first Enigma that used light bulbs for its output,
and is also known as
(glow lamp cipher machine machine).
It was intended as a portable and affordable alternative to the much larger
printing Enigma machine
Unlike the later Enigma B
and Enigma C – which were available with 28 or
29 keys – the Enigma A was only available with 26 keys (and hence 26
contact points on the rotors). In November 1924, the machine was in stock,
and was offered
at a price of ℛℳ 500. When buying 10 units or more, the price
was ℛℳ 420, and for orders of 50 units or more, the unit price dropped to
Several variants of the machine are known to have existed, none
of which seems to have survived.
The diagram below shows what the machine may have looked like, based on
descriptions found in the archives of the FRA
in Sweden, that were published by Anders Wik in 2018 . Apparently,
this machine had been shown publicly in August 1924 – on the
Universal Postal Union Congress in Stockholm  –
together with a bigger printing variant,
known as Die Handelsmaschine.
After the congress, both machines were borrowed for evaluation
by the Swedish General Staff (SGS).
The 'Stockholm' Enigma A
According to surviving documents, the Enigma A measured 27 x 23 x 13 cm
and had a weight of approx. 5 kilograms. It had 26 keys, divided over two rows.
Above each row, was a row with the corresponding light bulbs.
The keys were blank, so that any character could be written on the key top.
Likewise, the lamp panel was blank, so that any letter, number or punctuation
mark could be written on the opaque lamp panel cover. There were
two moving rotors — marked with numbers (01-26) — and a protruding settable
reflector 1 (left) that was marked with letters (A-Z) .
A disadvantage of the machine, was that the lamp panel was
interleaved with the keyboard. When pressing a key on the upper row,
the user could easily obscure one of the lamps of the lower row. This
was solved in the Enigma B,
where the entire lamp panel was placed behind the keyboard.
Before entering a letter, the user first had to push the large button at
the top right, in order to cause the rightmost rotor to make a single step.
After a key had been pressed, all keys were locked until the
Antriebstaste was pressed again.
Apparently, the actual keys were no more than simple electric switches.
The keyboard was very dense, and it is possible that in a later design,
the keys were divided over three rows, which might have provided room
for 28 keys instead.
It is likely that there were several variants of the Enigma A and that
the one shown in Stockholm was just a prototype. Different versions of the
machine may have existed before and after it.
As far as we know, there are no surviving examples of this machine.
It is likely that it was never built in a large quantity – probably no
more that 10 or 20 units 2 – before it was succeeded by the improved
Enigma B, which had more space for the keyboard
and did not require an Antriebstaste.
It is possible that the Settable UKW (i.e. the leftmost rotor) was not
present on the prototype that was shown at the Postal Congress in
August 1924. According to a letter from ChiMaAG
to the SGS on 14 November 1924,
this feature was added later .
According to an offer from manufacturer ChiMaAG
to the SGS in November 1924,
they had 10 units available for immediate delivery .
Below is the simplified circuit diagram of the Enigma A.
At the right are the keyboard switches and the lamps. At the left
are the two cipher rotors (I and II) mounted in between the entry
and the reflector (UKW).
Rotor stepping was controlled by the so-called Antriebstaste.
The circuit diagram shows that the cipher is symmetric (reversable) —
i.e. if A → B then B → A — and that a letter can never be
encoded into itself. Both effects are inherent properties of the UKW.
The machine had a cipher period of 676 (26 x 26), which means that the
UKW was not moving. It offered more than 17,000 settings
(probably 26 x 26 x 26 = 17576), which means that the UKW was
settable, meaning that it could
be set to any of 26 positions at the start of a message. 1
This feature was added later, and may not have been available in
the prototype of August 1924.
After the Postal Congress of August 1924 in Stockholm, the Swedish
General Staff (SGS) borrowed two machines from the Enigma manufacturer
– ChiMaAG – for evaluation:
a large printing Enigma
– Die Handelsmaschine –
and the much smaller Enigma A, also known as kleine Militärmaschine.
Once the evaluation period was over — in September 1924 — both machines
were returned to the manufacturer in Berlin, followed in October by a list
of desired modifications for the latter:
- A possibility to test all lamps
- Lamp panel and keyboard should be printed with letters
- Lamp panel should have white letters on a dark background
- The Antriebstaste (driving key) should be moved to the left side
In addition, the following improvements would have been nice:
- Larger character set (~40) with letters, numbers and punctuation marks
- Four cipher wheels (instead of two)
In the choice between the Enigma A and an improved model, called
the SGS chose the latter where much of the imperfections of the Enigma A
had been fixed, including modifications suggested by the SGS. Two
Enigma B machines were ordered in January 1925. When they were delivered
in April 1925, the SGS got an
improved version of the Enigma B —
with changes that had meanwhile been agreed —
with a layout closer to the (now well-known) Enigma design.
➤ More about Enigma B
- DE407804 / 18 Jan 1924
First patent in which a cipher machine with light bulbs (Glühlampen) is
Invented by Paul Bernstein. Filed by Chiffriermaschinen AG, Berlin.
Although the model shown in the patents does not yet have a reflector
(UKW), the keyboard and the lamp panel are very similar to those
of the Enigma A, as described above.
- DE460457 / 11 March 1926
This patent introduces the Umkehrwalze (UKW) and the
removable rotor-set, or drum, invented by Willi Korn.
It describes how the drum can be removed by using a lever
to shift the UKW aside. This was done
to allow the wheel order to be changed easily in the field.
It also claims that the top lid can only be closed when the
UKW lever is locked in position.
The UKW is a basic element for all
glow lamp Enigma machines.
The drawing below was taken from this patent
and shows a construction that is very similar to the
later Enigma B.
Document courtesy Anders Wik .
Document courtesy Anders Wik .
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© Crypto Museum. Created: Tuesday 29 January 2019. Last changed: Sunday, 14 June 2020 - 21:07 CET.