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Enigma Tree Handelsmaschine → Enigma A →
Early Enigma prototype
Enigma-Probemaschinen (English: Enigma test machines) were early
prototypes of a rotor-based cipher machine,
developed from 1918 onwards by German inventor Arthur Scherbius.
They led to the development of two separate lines of the
Enigma cipher machine: (1) the
Glühlampenchiffriermaschinen
(glow lamp cipher machines) and (2) the Schreibenden Maschinen
(printing machines).
As far as we know, there are no surviving examples of Scherbius' early
prototypes of the Enigma.
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The image below shows what the control panel of the Probemaschine might have
looked like, based on the drawings in the
patent description [2].
At the left are 25 keys arranged in a 5 × 5 matrix, printed with
the letters A-Z (note that the letter 'J' is omitted). At the right are
25 flat-faced lamps, also arranged in a 5 × 5 matrix. The lamps
represent the output of the cipher.
It is possible that the lamps were covered by a film on which the letters
A-Z were printed, but this is not certain. Move the mouse over the
drawing to see what it may have looked like in that case. In the drawing
above, the user has pressed the letter 'G', whilst the letter 'T' lights up.
The reason for the missing letter 'J' is probably that with 25 letters the
keys can be nicely arranged in a square matrix, which is not possible with 26
letters. It was a common arrangement that was also used by other cipher systems.
In the text, the letter 'J' can easily be replaced by the letter 'I'.
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The first prototype was made in April 1918 and had two rotors with 25
contacts each, resulting in 252 = 625 different alphabets.
It was clear to Scherbius however that this was weak, and that the
strength of the cipher had to be increased by adding more rotors.
During a demonstration for the German Navy on 10 May 1918 he gave the
following examples for different numbers of rotors [3]:
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Rotors
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Alphabets
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Notation
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2
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625
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252
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7
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6,103,515,625
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6.1 × 109
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10
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95,367,431,640,625
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9.53 × 1013
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12
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59,604,644,775,390,625
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5.96 × 1016
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Shortly afterwards, at the request of the German Navy, Scherbius demonstrated
another prototype with 7 rotors. Although the demonstration went well, he
soon experienced contact problems, that were probably caused by corrosion and
mechanical friction between the rotors [3]. It caused Scherbius to drop his
initial goal of using a large number of rotors, and settle for just
three or four, which resulted in the following – less astronomical –
number of different alphabets:
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Rotors
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Alphabets
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Notation
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3
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15,625
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1.56 × 104
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4
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390,625
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3.9 × 105
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In the following years, Scherbius and his staff would try to find other ways
to strengthen the cipher, for example by adding additional rotors to choose
from, swapping the rotor positions, adding a reflector and adding a plugboard,
altough not all of these had the desired effect.
The reflector – known as the Umkehrwalze or UKW – was invented by
Scherbius' employee Willi Korn and was first introduced on Enigma A.
It greatly simplified the mechanical and electrical construction of the machine
and its operation. It was thought to improve the strength of the cipher, as
the current passed through each of the three rotors twice, giving the
equivalent of six rotors. In reality, the fact that the current ran through
each wheel twice had no effect on the strength on the cipher. The UKW
even reduced the strength of the cipher, as it prevented a letter
from being enciphered as itself, a flaw that was ecploited during WWII by
British codebreakers.
The cipher strength of the Military Enigma I — developed between
1927 and 1930 — was improved by adding a plugboard (Steckerbrett). Although
the initial design did indeed significantly improve the strength, the final design
that was suggested by the Reichswehr (later: Wehrmacht) was much weaker and
self-reciprocal, as a result of which it became easier for the British
codebreakers to attack the machine [3].
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Below is the simplified circuit diagram of the first prototype, based on the information
given in the original patent [2].
To show the principle, only the letters A-F are shown here.
At the top are the two rotors (I, II),
located in between two contact discs (ETW1, ETW2).
At the bottom are the keys A-F, used for input, and the lamps A-F, used for output.
At the center is a commutator that can swap the wiring of ETW1 and ETW2. It is used
for selecting between CIPHER and DECIPHER.
The diagram above shows the Probemaschine in CIPHER mode.
When pressing the letter 'F', the current flows from the battery through switch 'F'
on to the commutator. From there it flows through ETW1, rotor I, rotor II and ETW2, until it enters the right side of the commutator. From the commutator
the current flows to the lamp panel where in this case the letter 'E' lights up.
Move the mouse over the diagram to see what happens when the device is switched
to DECIPHER.
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© Crypto Museum. Last changed: Friday, 23 August 2024 - 10:31 CET.
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