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Enigma B
Glow lamp Enigma machine · 1924-1925

Enigma B, was an electro­mechanical rotor-based cipher machine, introduced in late 1924 by Chiffriermaschinen Aktiengesellschaft (ChiMaAG) in Berlin (Germany). Like its predecessor, the Enigma A, it used light bulbs for its output, and was therefore known as a Glühlampen­chiffrier­maschine (glow lamp cipher machine). It was short-lived, and rapidly evolved into the Enigma C.

Two version of the Enigma B are known: (1) an early one with two rows of keys at the front and two rows of lamps behind the wheels, and (2) a later one with three rows of keys at the front, the lamps at the centre and the wheels at the back.

The first model has two (fixed in place) cipher wheels and a moving reflector (
UKW
). The later model has three cipher wheels plus a fixed
UKW
. The later one is also the first model of which the cipher wheels are removable (allowing the order to be changed), and have an
adjustable ring
. The image on the right shows the later model B.
  
Later model of the Enigma B (serial number A-133) as bought by the Swedish SGS in 1925 [2]. Photographed in 2009, courtesy FRA [3].

Enigma B is also the first Enigma machine of which the rightmost rotor is advanced automatically when a key is pressed. For this reason, the
Antriebstaste
– present on the Enigma A – is missing. Note that the keyboard is not arranged as a regular typewriter, but in the order of the alphabet.

It is believed that there are no surviving examples of the early model, and it is likely that only a small quantity was ever built. There are at least two surviving examples of the later model (A-133 and A-134), that are now part of the collection at the Swedish FRA [3]. The differences between the two versions are so large, that the later one might be seen as a cross-over to the Enigma C.

Swedish Enigma B variant, seen from the front left Swedish Enigma B variant, seen from the front Swedish Enigma B variant, seen from the front right Swedish Enigma B variant, seen from the front right Cipher wheels Close-up of the lock
A
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A
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Swedish Enigma B variant, seen from the front left
A
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Swedish Enigma B variant, seen from the front
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Swedish Enigma B variant, seen from the front right
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Swedish Enigma B variant, seen from the front right
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Cipher wheels
A
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Close-up of the lock

Differences with Enigma A
Compared to its predecessor – the Enigma A – the following differences can be observed:

  • Automatic advancing of the rotors (no
    Antriebstaste
    )
  • Three moving cipher wheels
  • Fixed reflector
  • Removable cipher wheels
  • Movable letter ring (Ringstellung)
  • Keys and lamp panel printed with letters
  • Lamp panel white text on black background
  • Single-piece top panel
  • Lamp test function
Enigma B — Mark I
The first version of the Enigma B was available in November 1924, and is described in some detail in correspondence between the manufacturer
ChiMaAG
and the Swedish General Staff (SGS) of November 1924 [1]. From front to back, there are 26 keys, divided over two rows, three cipher wheels, of which the leftmost one is a moving reflector (
UKW
), and 26 lamps, printed with the alphabet (A-Z) in white on a dark background. When pressing a key, the rightmost rotor moves to the next position before the contacts of the switch (below the key) are closed and a lamp is lit.

Educated guess of the front panel of the early Enigma B, as it was offered to the SGS in November 1924
The proposed Enigma B with 26 keys and 26 lamps

The cipher period of this machine is described as approx. 17,500 (26 x 26 x 26 = 17576), which indicates that it has a moving reflector (
UKW
). It is unclear whether the two cipher wheels of this model are removable or not. If they are fixed, the number of settings would be 17576, but when they can be removed (and hence can be swapped), the number of settings would be 35152.

Simplified circuit diagram of the early version of the Enigma B

The diagram above shows the simplified circuit diagram of the Enigma B, which is very similar to that of the Enigma A, except that its reflector (
UKW
) can be driven by the wheel to its right. This makes the cipher period much longer (17,576 instead of 676). Like with all Enigma machines that have a reflector, the process is reversable (i.e. encoding is identical to decoding), but a letter can never be encoded into itself. This property should be considered a weakness of the cipher.

Educated guess of the front panel of the early Enigma B, as it was offered to the SGS in November 1924
Proposed modified Enigma B with 28 keys and 28 lamps

According the correspondence [5], this version could also be supplied with 28 keys (rather than 26), but it does not say how the keys would be arranged in that case. The drawing above provides an educated guess of what the machine may have looked like. It is clear that it would have been wider than the 26-key version. Although the 28-key version was offered to the
SGS
in November 1924, and subsequently ordered by the
SGS
in January 1925, it was never delivered in this form, as the design had meanwhile been altered considerably, with the approval of the
SGS
(below).

Lamp test
The machine offers the ability to check all lamps, which was probably done as in later machines: by holding down a key (to prevent the rotors from moving), then pressing and holding another key and releasing the first one, and so on, until all keys (and hence all lamps) have been tested.


Enigma B — Mark II
On 13 November 1924,
ChiMaAG
proposed two machines to the Swedish General Staff (SGS): the Enigma A, which was in stock, and an improved machine – Enigma B – that could be made with 28 keys rather than 26. The latter was not in stock and had to be built to order, which would take approx. two months. The unit price for low quantities of the machine was ℛℳ 650. When buying 10 units or more, the price would drop to ℛℳ 490, and for 50 units or more, the unit price was as low as ℛℳ 450. In January 1925, the
SGS
placed an order for two such Enigma B machines [5].

The machines were not ready in time, but were reportedly delayed due to the suggestions made by Captain Gyllencreutz of the
SGS
, probably related to the addition of an extra cipher wheel and the use of Swedish letters on the keyboard [1]. In addition, the position of the wheels and the lamp panel was altered. The changes and improvements were probably merged with the ongoing developments at
ChiMaAG
at the time, and were carried out with the consent of the
SGS
.

Enigma B as delivered to the SGS in April 1925
Enigma B with 28 letters as delivered to the SGS in April 1925

Finally, in April 1925, two machines of the improved design were delivered to the
SGS
, with serial numbers A-133 and A-134 respectively [2]. The changes that were made to the original design of the Enigma B, are significant. The keys of the keyboard have been spread over three rows, so that their operation becomes easier, but they are still arranged in the order of the alphabet. The lamp panel is now at the centre of the machine whilst the wheels have been relocated towards the rear.

Lower row of keys on the keyboard of the Swedish Enigma B

Note that the letter 'W' is missing, whilst the extra letters Å, Ä and Ö have been added at the bottom right. The letter 'W' was not used in official Swedish language at the time, except for names, foreign words and loan words, in which case it could be replaced by 'VV' (double-V).

Simplified circuit diagram of the later version of the Enigma B


Inside the A-133
The above machine was shown by the FRA at the Enigma Reunion of 2009 at Bletchley Park (UK). It has three moving cipher wheels — one of which has letters (A-Ö), whilst the other two have numbers (01-28) — and a fixed reflector (
UKW
) fitted inside the machine, to the left of the cipher wheels. The wheels can be removed from the spindle, and can be placed in 6 (3!) different orders (3 x 2 x 1) which increases the key space to 28 x 28 x 28 x 6 = 131,712 possible settings. As each cipher wheel can be driven by a key-press, the cipher period is 28 x 28 x 28 = 21,952.

Enigma B (A-133) with its metal cover removed. Photograph courtesy FRA [2].

Furthermore, the position of the engraved rings (with letters or numbers) of each wheel, can be set to any of 28 positions. Although this greatly increases the total numbers of possible settings (2,891,341,824 instead of 131,712), it does not affect the cryptographic strengh of the cipher in any way, as the ring is not attached to the position of the turnover notch, relative to the wiring of the wheel. This shortcoming was first fixed in 1927 in the Enigma K.

The ring-setting (German: Ringstellung) and wheel order (Walzenlage) were intended as a so-called inner key, whilst the start position of the wheels was used as the outer key. By using a secret inner key for a specific organisation, an eavesdropper who had obtained a valid outer key, would not be able to decode any traffic, if he did not have access to the secret inner key as well.

It is believed that the later version of the Enigma B (shown above) is very similar to its successor, the Enigma C, which was introduced shortly afterwards. In fact,
ChiMaAG
was already working on a similar machine for the German Navy, which would become known as Funkschlüssel C.

Swedish version of the Enigma B (check the keyboard) Swedish version of the Enigma B (A-133) Close-up of the rotors and part of the lamp panel Close-up of the lock and some keys Enigma B (A-133) with its metal cover removed. Photograph courtesy FRA [2].
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Swedish version of the Enigma B (check the keyboard)
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Swedish version of the Enigma B (A-133)
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Close-up of the rotors and part of the lamp panel
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Close-up of the lock and some keys
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Enigma B (A-133) with its metal cover removed. Photograph courtesy FRA [2].

Wiring of the A-133
Below is the wiring of the Swedish Enigma B with serial number A-133, as it was delivered to the
SGS
on 6 April 1925 [2]. The keyboard, the lamp panel and the wheels, each have 28 positions. Note that rotor I has letters on the ring (A-Ö), whilst rotors II and III have numbers (01-28). The reason for this is unknown, but it was probably done for consistency with the earlier version. To make the table more uniform, we have translated the numbers into letters, using this scheme:

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
ABCDEFGHIJKLMNOPQRSTUVXYZŠĠ
Note that the letter 'W' is missing and that the letters Å, Ä and Ö are added at the end, after the letter 'Z'. When needed, the letter 'W' (hardly used in Swedish) was replaced by 'VV' (double-V).

Wheel ABCDEFGHIJKLMNOPQRSTUVXYZÅÄÖ Notch Turnover #
ETW 2 ABCDEFGHIJKLMNOPQRSTUVXYZÅÄÖ      
I PSBGÖXQJDHOÄUCFRTEZVÅINLYMKA G Ä 1
II 1 CHNSYÖADMOTRZXBÄIGÅEKQUPFLVJ G Ä 1
III 1 ÅVQIAÄXRJBÖZSPCFYUNTHDOMEKGL G Ä 1
UKW LDGBÄNCPSKJAVFZHXUIÅRMQÖOTEY      
  1. This wheel has numbers (01-28) rather than letters (A-Ö).
  2. The wiring of the ETW has not yet been verified.

Related patents
  • 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 patent DE460457 and is very similar to the later version of the Enigma B.

    Click to view patent DE460457
Glossary
ETW   Eintrittswalze
Entry disc with 26 (or 28) contacts, that connects the keyboard switches to the rightmost contacts of the first rotor (from the right).
SGS   Swedish General Staff
UKW   Umkehrwalze
Reflector, fitted at the left end of the Enigma rotor stack, with 26 (or 28) contacts, that are interconnected in pairs.
Walze   Wheel
German word for a cipher wheel, or rotor.
Documentation
  1. Bedienungsvorschrift für die Glühlampenchiffriermaschine 'Enigma'
    Operating instructions for the glow lamp cipher machine Enigma (German). 1,2
    Date unknown. 2 pages.
  1. Document courtesy Anders Wik [1].
  2. These operating instructions are probably related to the later version of the Enigma B.

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

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

  3. Försvarets radioanstalt (FRA), Enigma B, S/N A-133
    Bletchley Park, UK, September 2009.

  4. Frode Weierud, Enigma History
    Personal correspondence, 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 [1].

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Crypto Museum. Created: Tuesday 29 January 2019. Last changed: Saturday, 16 February 2019 - 14:10 CET.
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