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Enigma Steckerbrett
Single-ended and double-ended plugboards

The Enigma I, Enigma M1, M2, M3 and Enigma M4, are the only models that have a Steckerbrett (plugboard) at the front. The Steckerbrett has 26 sockets that are marked with the letters of the alphabet (A-Z), the numbers 1-26 or both. Each socket accepts a plug with two pins: a thick one and a thin one. The 2-wire cable 1 between two plugs is cross-wired, as a result of which letters are always swapped in pairs. Although this presents a weakness, it was done for good reasons.

The Steckerbrett was manufactured exclusively for the German Armed Forces and was patented by the Reichswehr (later: Wehrmacht), which is why it was not available to other Enigma users.

The Steckerbrett as we know it however, was not the first and only design that was tried. Various designs were tried before the Reichswehr chose the double-ended variant that was used during WWII. Note that this version is not compatible with any of the earlier designs. The image on the right shows the Steckerbrett of the Enigma I, which is nearly identical to that of the Naval one.
  
Removing a plug (Stecker)

This page provides some information about the development of the Steckerbrett, and provides 'educated guesses' of what the earlier designs may have looked like. We should like to thank Frode Weierud for providing the historical documents that have made this page possible [1].

  1. Note that the pins of the plugs of the Naval Enigma (M1, M2, M3 and M4) are longer than those of the Service Enigma (Enigma I), and are therefore not interchangeable.

Enigma A598 Enigma A598 Steckerbrett Enigma A598 with dismounted Steckerbrett Front perspective view, showing two large circular cut-outs to fit the original (single-ended) Steckerbrett Front view, showing two circular cut-outs Cut-out in the bottom casting Cut-out in the casting of the keyboard Cut-out in the casting of the keyboard
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Enigma A598
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Enigma A598 Steckerbrett
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Enigma A598 with dismounted Steckerbrett
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Front perspective view, showing two large circular cut-outs to fit the original (single-ended) Steckerbrett
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Front view, showing two circular cut-outs
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Cut-out in the bottom casting
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Cut-out in the casting of the keyboard
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Cut-out in the casting of the keyboard

Initial version   Mark 1
It has been known for some time, that the initial version of the Steckerbrett (plugboard) of the military Enigma machine, was very different from the later double-ended design that was used throughout WWII [2]. It was likely that single-ended patch cables were used in the initial design.

The exact layout remained a mystery however, mainly because documents describing the early design have not been recovered and no surviving machines with the early design have been found.

In October 2009, whilst restoring Enigma A604, researcher Tom Perera noticed that the casting of the bottom plate and the keyboard panel had circular cut-outs, that apparently had been used at some point [3]. They are normally invisible, as they are hidden behind the Steckerbrett. After checking Enigma A598, Crypto Museum was able to confirm that it has the same circular cut-outs.
  
Front perspective view, showing two large circular cut-outs to fit the original (single-ended) Steckerbrett

The image above shows the front side of Enigma A598, after the plugboard has been removed. The cut-outs are clearly visible here. When extended to a full circle, each cut-out has a diameter of exactly 8 cm, and was clearly designed to accomodate a circular device. It has been speculated that perhaps it was meant for two permutation switches, or a storage space for two spare wheels.

But after carefully studying the possibilities and the available documentation — quoted in 2016 in a Cryptologia article about UKW-D by Olaf Ostwald and Frode Weierud [4] — it has meanwhile become clear that the cut-outs were most likely intended for the initial design of a single-ended Steckerbrett that held 26 sockets, arranged in two circles of 13 sockets each. It is very likely that standard banana-sockets and banana-plugs were used, and that the Steckerbrett looked like this:


In this design, 26 patch cables are present: 13 at the left and 13 at the right. One end of each patch cable is fixed inside the machine, whilst the other end has a 4 mm banana-plug. The sockets on the left are marked with the numbers 1-13, and the ones on the right carry the numbers 14-26, engraved on the panel. The banana-plugs are marked with the letters of the alphabet: A - M on the left, and N - Z on the right, all engraved in the isolating grip of the plug. The patch cables were short, so that each plug could only be fitted in a socket of its own circle.


The image above gives an impression of what it might have looked like, when the plugboard was configured with the unity matrix (A-1, B-2, C-3, ... etc.). Although this design offers a nice and clean solution, it clearly limits the maximum number of permutations, as each plug is restricted to its own half. This is confirmed by a document that was discovered by Frode Weierud [4], in which the number of possible permutations is defined as 13 factorial multiplied by 13 factorial:

13! × 13! = 6227020800 × 6227020800 = 38,775,788,043,630,000,000


Improved version   Mark 2
400 machines with the Mark 1 Steckerbrett had already been supplied, when the Reichswehr disapproved the design. It was probably seen as a weakness that the cables from the left circle could not be patched to the sockets of the right circle. In an attempt to improve the Steckerbrett, the manufacturer — Chiffriermaschinen AG — converted the Steckerbrett of Enigma A366 in such a way that the maximum number of permutations was obtained. This designed looked like this:


With this version, the Steckerbrett has 52 sockets, organised as 4 rows of 13 sockets each. The upper two rows are marked with the numbers 1-13 and 14-26. The lower two rows are marked with the letters of the alphabet: A-M and N-Z. The machine came with 26 patch cables that each had a banana-plug at either end, and that were long enough to reach the sockets furthest away (e.g. N to 13). The plugs were no longer engraved with the letters of the Latin alphabet [4].


It is mandatory that all cables are installed on the Steckerbrett, and that connections are only made between the upper two and the lower two rows, which is prone to mistakes. In practice this must have been a nightmare. From a cryptographic point of view however, this is by far the best solution, as it provides the maximum number of permutations, which is calculated as follows:

26! = 403,291,461,126,600,000,000,000,000

On 14 February 1928, the modified Enigma with serial number A366 was demonstrated in Berlin at the cipher bureau of the German High Command, OKW Chi. Although Councilor Fenner was able to populate the Steckerbrett according to a table in less then three minutes, the opinion of the participants 1 was that 26 long cables and 52 plugs on the plugboard was too complicated.

In particular the fact that sockets from the upper two rows must only be patched to sockets on the lower two rows — i.e. letters had to be patched to numbers — was seen as a potential cause of mistakes. Furthermore, it was deemed necessary to extend the length of the wooden transport case by 5 or 6 cm, in order to accomodate all the cables. As a result, the design was rejected [4].

Interestingly, this approach was reused by the Russians shortly after WWII on their M125 Fialka cipher machine, where it was implemented as a sliding card reader at the machine's left side.
  
An original Fialka key card in the card reader

The card reader is in fact a 30 x 30 matrix of switches 2 that are controlled by a punched key card that is inserted into the drawer. When the drawer is closed, the holes in the key card slide 30 movable contacts into position. Like Enigma's Steckerbrett, the card reader is connected between keyboard and entry disc. But unlike on Enigma, the card is installed in seconds without mistakes.

  1. Present at the meetings of 14 and 17 February 1928 at the Reichswehrministerium (Ministry of Defense) in Berlin, were Oblt. Seiffert, Regierungsrat Fenner, Major Schröder, Frau Rinke and Herrn Korn. The latter two represented Chiffriermaschinen AG.
  2. As the Russian (Cyrillic) alphabet has more letters than the Latin one, Fialka has 30 letters, whereas Enigma has 26. As a result the permutation matrix has 30! possible combinations, rather than Enigma's 26!.

Final version   Mark 3
After the Mark 2 design had been rejected, the Reichswehr came up with a simplified design in which just 26 sockets are used. Each socket has two contacts, and accepts a custom plug with two pins. The sockets are self-closing, which means that if no plug is inserted, the two contacts are automatically shorted by an internal spring-loaded shorting bar fitted behind the socket.


Each machine was supplied with 12 patch cables, each of which had two wires and was approx. 20 cm long. At either end was a black bakelite plug with two pins — a thick one (4 mm) and a thin one (3 mm) — that were cross-connected to the pins of the other plug, as shown here:

Enigma patch cable. Click to see a photograph.

Because the two pins have a different diameter, the plug cannot be inserted the wrong way around. The diagram below shows a cross-section of the final 1928 Steckerbrett design. For simplicity, we are only showing the sockets corresponding to the letters A (01) and Y (25). The left half of the diagram shows the situation when no plugs are inserted into the sockets and the shorting bar connects the two contacts of each socket. The right half shows what happens when both plugs are inserted. The shorting bar is pushed out of the way, and the plugs take over.

Cross-section of the Enigma Steckerbrett

Klappe schließen — Note that for a proper operation of the Steckerbrett, all plugs must be fully inserted into the sockets, or else the shorting bar may not be pushed out of the way, leading to internal short-circuits. As a result, more than one lamp may be lit when a key is pressed. For this reason, the wooden case is constructed in such a way that the hinged flap at the front can only be closed when all plugs are fully inserted. This is also the reason why most machines have the text Klappe schließen (close flap) printed on the inside of the flap.
All 400 machines, that had already been delivered with a Mark 1 Steckerbrett, were subsequently returned to the factory where they were reworked into a Mark 3. It is therefore likely that there are no surviving machines with a Mark 1 Steckerbrett. Sometime later, the molds were modified as well, so that the casting of later machines no longer had the circular cut-outs at the front.

The new design accepts any number of cables with a minimum of 0 and a maximum of 13. The designers thought that in practice 6 would be sufficient. During the war, 10 patch cables 1 were most commonly used on the Steckerbrett, with 2 spare ones stored in the lid of the wooden case.

With the early machines, the Steckerbrett panel is engraved with both letters and numbers, which is no longer necessary as there is no difference between upper and lower sockets anymore. This was probably inherited from the earlier Mark 2 design. It was later dropped in favour of numbers.

Although the new Steckerbrett design is much cleaner and less prone to mistakes, the number of possibilities is far less astronomical than with the two earlier designs. Because letters are always swapped in pairs, the Steckerbrett is said to be self-reciprocal 2 . The total number of possible combinations (N) depends on the number of patch cables (n) and is calculated with the formula:


If the number of patch cables had been variable (anything from 0 to 13 cables), the total number of permutations would have been 532,985,208,200,000. It was decided however, to always have a fixed number of cables on the Steckerbrett, which reduced the actual number of combinations. Initially 6 patch cables were used, but most networks later used 10 cables, which gives us:

150,738,274,900,000


The table below gives the number of permutations for each number of patch cables [5]:

Cables (n) Possible combinations (N)
0 1
1 325
2 44,850
3 3,453,450
4 164,038,875
5 5,019,589,575
6 100,391,791,500
7 1,305,093,290,000
8 10,767,019,640,000
9 53,835,098,190,000
10 150,738,274,900,000 <-- Most common number of cables 1
11 205,552,193,100,000
12 102,776,096,500,000
13 7,905,853,580,550
Total 532,985,208,200,000
  1. From the table it is evident that the maximum number of combinations is obtained when using 11 cables rather than 10, but it is possible that this was not known at the time.
  2. The Steckerbrett is self-reciprocal, which means that if A is patched to Y, then Y is automatically patched to A. This greatly reduces the number of permutations, and was exploited during WWII by the codebreakers at Bletchley Park. When determining the order of the cipher wheels for a given day, the Steckerbrett could be eliminated from the equasion completely.

References
  1. Frode Weierud, Personal correspondence
    October 2017.

  2. Olaf Ostwald & Frode Weierud (2015),
    History and Modern Cryptanalysis of Enigma's Pluggable Reflector
    Cryptologia, Volume 40, Issue 1, January 2016, pp. 70—91.

  3. Tom Perera, Personal correspondence
    October 2009 - October 2017.

  4. Korn & Rinke, Aktennotiz 14.2. und 17.2. 1928
    Notes for the record about meetings held on 14 and 17 February 1928.
    17 February 1928. Kindly provided by [1].

  5. Arthur Bauer, Funkpeilung als alliierte Waffe gegen Deutsche U-Boote 1939-1945.
    ISBN: 3-00-002142-6. January 1997. p. 33.
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Crypto Museum. Created: Wednesday 11 October 2017. Last changed: Saturday, 09 June 2018 - 21:59 CET.
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