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Enigma A
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 Glühlampenchiffriermaschine (glow lamp cipher machine machine). It was intended as a portable and affordable alternative to the much larger printing Enigma machine [1].

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 ℛℳ 400 [5]. 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 [2]. Apparently, this machine had been shown publicly in August 1924 – on the Universal Postal Union Congress in Stockholm [4] – 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).

Educated guess of the front panel of the Enigma A, as it was shown on the Postal Congress in Stockholm in August 1924. Copyright Crypto Museum 2019.
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) [2].

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.

  1. 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 [5].
  2. According to an offer from manufacturer ChiMaAG to the SGS in November 1924, they had 10 units available for immediate delivery [2].

Circuit diagram
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 disc (ETW) 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.

Simplified circuit diagram of the Enigma A

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

  1. This feature was added later, and may not have been available in the prototype of August 1924.

Swedish General Staff
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 EnigmaDie 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 Enigma B, 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

Related patents
  • DE407804 / 18 Jan 1924
    First patent in which a cipher machine with light bulbs (Glühlampen) is described. 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.
    Click to view patent DE407804

  • 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.

    Click to view patent DE460457
  1. Glühlampenchiffriermaschine 'Enigma A'
    Brief description of the Enigma A (German). 1
    ChiMaAG. Berlin, 1 August 1924. 1 page.
  1. Document courtesy Anders Wik [2].

  1. Frode Weierud, Enigma History
    Personal correspondence, January 2019.

  2. Anders Wik, The First Classical Enigmas,
    Swedish Views on Enigma Development 1924-1930
    HistoCrypt 2018, Proceedings. 18-20 June 2018, pp. 83-88.

  3. Försvarets radioanstalt (FRA) archive
    1924-1930. Bearbetningsbyrån F V:1. Chifferapparaten Enigma.

  4. Wikipedia, Postal Union Congress
    Retrieved January 2019.

  5. Letter from ChiMaAG to the Swedish Military Attaché in Berlin
    Offering of Enigma A and Enigma B. 13 November 1924. 4 pages. 1
  1. Document courtesy Anders Wik [2].

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Crypto Museum. Created: Tuesday 29 January 2019. Last changed: Sunday, 14 June 2020 - 21:07 CET.
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