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Rotor Enigma Tree C → ← A
Two version of Enigma B are known: (1) an early one with two
rows of keys at the front and two rows of lamps behind the rotors,
and (2) a later one with three rows of keys at the front, lamps
at the centre and the rotors towards the rear.
The first model has two (fixed in place) cipher rotors and a moving
reflector (UKW). The later model has three cipher rotors plus a fixed
UKW.
The later one is also the first model of which the cipher rotors
are removable (allowing the order to be changed), and have an
adjustable ring. The image on the right shows the
later model B.
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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), both of which are in the collection of 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.
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Differences with Enigma A
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Compared to its predecessor – the Enigma A
– the following differences can be observed:
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- Automatic advancing of the rotors (no Antriebstaste)
- Three moving cipher rotors
- Fixed reflector
- Removable rotors
- 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
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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
rotors, 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.
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The proposed Enigma B with 26 keys and 26 lamps
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The cipher period of this machine is described as approx. 17,500
(26 x 26 x 26 = 17,576), which indicates that it has a moving
reflector (UKW). However, as the middle rotor suffers from the
so-called double-stepping anomaly [6], the cipher period is reduced
to 26 × 25 × 26 = 16,900.
This also reduces the key space to 16,900 as the remaining settings
are redundant.
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 rotor to its right. This
makes the cipher period much longer (16,900 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.
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Proposed modified Enigma B with 28 keys and 28 lamps
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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).
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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.
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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 rotor and the use of Swedish letters on
the keyboard [1]. In addition, the position of the rotors 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.
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Enigma B with 28 letters as delivered to the SGS in April 1925
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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 rotors have been relocated towards the rear.
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).
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The above machine was shown by the FRA at the
Enigma Reunion of 2009
at Bletchley Park (UK).
It has three moving cipher rotors — 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 rotors.
The rotors 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 27 x 28 x 6 = 127,008 possible settings. 1
As each cipher rotor can be driven by a key-press, the cipher period is
28 x 27 x 28 = 21,168.
Furthermore, the position of the engraved rings (with letters or numbers)
of each rotor, can be set to any of 28 positions.
Although this greatly increases the total number 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 rotor.
This shortcoming was first fixed in 1927 in the Enigma K.
The ring-setting (German: Ringstellung) and rotor order (Walzenlage)
were intended as a so-called inner key, whilst the start position of
the rotors 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 — 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.
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Although in theory there are 28 × 28 × 28 = 21,952 possible
settings of the rotors, the middle one suffers from the so-called
double-stepping anomaly, as a result of which it has only 27
useful settings [6]. For the key space (with 6 possible rotor orders)
this means that the cipher period is 127,008 instead of 131,712.
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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 rotors, 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:
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).
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Rotor
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ABCDEFGHIJKLMNOPQRSTUVXYZÅÄÖ
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Notch
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Turnover
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#
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ETW 2
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ABCDEFGHIJKLMNOPQRSTUVXYZÅÄÖ
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I
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PSBGÖXQJDHOÄUCFRTEZVÅINLYMKA
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G
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Ä
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1
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II 1
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CHNSYÖADMOTRZXBÄIGÅEKQUPFLVJ
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G
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Ä
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1
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III 1
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ÅVQIAÄXRJBÖZSPCFYUNTHDOMEKGL
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G
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Ä
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1
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UKW
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LDGBÄNCPSKJAVFZHXUIÅRMQÖOTEY
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This rotor has numbers (01-28) rather than letters (A-Ö).
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The wiring of the ETW has not yet been verified.
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- 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 rotor 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.
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ETW
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Eintrittswalze
Entry disc with 26 (or 28) contacts, that connects the keyboard switches
to the rightmost contacts of the first rotor (from the right).
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SGS
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Swedish General Staff
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UKW
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Umkehrwalze
Reflector, fitted at the left end of the Enigma rotor stack, with 26
(or 28) contacts, that are interconnected in pairs.
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Walze
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Rotor
German word for a cipher wheel, or rotor.
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Price depending on ordered quantity. 2 @ ℛℳ 650, 10 @ ℛℳ 490,
50 @ ℛℳ 450 [5].
Prices are in Reichsmark (ℛℳ) — the German currency of the era.
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Document kinkly provided by Anders Wik [1].
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These operating instructions are probably related to the later
version of the Enigma B.
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- Anders Wik, The First Classical Enigmas,
Swedish Views on Enigma Development 1924-1930
HistoCrypt 2018, Proceedings. 18-20 June 2018, pp. 83-88.
- Försvarets radioanstalt (FRA) archive
1924-1930. Bearbetningsbyrån F V:1. Chifferapparaten Enigma.
- Försvarets radioanstalt (FRA), Enigma B, S/N A-133
Bletchley Park, UK. Enigma Reunion, September 2009.
- Frode Weierud, Enigma History
Personal correspondence, January 2019.
- Letter from ChiMaAG to the Swedish Military Attaché in Berlin
Offering of Enigma A and Enigma B. 13 November 1924. 4 pages. 1
- Enigma: Actions Involved in de 'Double Stepping' of the Middle Rotor 2
David H. Hamer, January 1997.
Cryptologia, Volume XX, Number 1.
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Document kindly provided by Anders Wik [1].
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Document reproduced by kind permission from the author (Author's copy).
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© Crypto Museum. Created: Tuesday 29 January 2019. Last changed: Thursday, 08 August 2024 - 08:02 CET.
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![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].](../img/300699/005/full.jpg)
![Enigma B (A-133) with its metal cover removed. Photograph courtesy FRA [2].](img/a133_open_large.jpg)
![Enigma B (A-133) with its metal cover removed. Photograph courtesy FRA [2].](img/a133_open_thumb.jpg "image # a133_open_large.jpg")
