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The Service Enigma Machine
Enigma I (Roman number 1) was the first lamp-based
Enigma machine that was
used exclusively by the German Army before and during WWII.
It was based on the chassis of the Enigma D but had a
fixed reflector (UKW) and a unique plug board (Steckerbrett) behind a wooden
flap at the front. The machine is also known as Service Enigma
or by its official designator Ch.11a.
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Initially, the machine was supplied with three coding wheels, that could
be inserted in any of 6 possible orders (3 x 2 x 1).
In December 1938, two additional wheels were supplied,
bringing the total number of possible wheel orders to 60 (5 x 4 x 3),
a 10-fold increase in cipher security!
It is however the Steckerbrett that increased the total number of
combinations dramatically.
The image on the right shows a typical Enigma I that was used by the
German Army. It was found as lost luggage in a train in Italy at the
end of WWII. It shown here with the top lid and the flap open.
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The Enigma I was used by both the Wehrmacht (Army) and the
Luftwaffe (Airforce).
It was later adopted by the Kriegsmarine (German Navy) where it
became known as the M1, M2 and finally the
M3.
The only obvious differency between the Army version and the
Navy version is that the wheels of the latter have letters (A-Z)
rather than numbers.
About 20,000 machines of this type were manufactured by various
manufacturers, but only a handful have eventually survived.
 Enigma working principle
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The machine is known by the following names:
- Reichswehr Enigma
- Wehrmacht Enigma
- Heeres Enigma
- Ch.11a
- Enigma I
- Army Enigma
- Service Enigma
- Army/GAF Enigma
- 3-wheel Enigma
- 3-wheeler
The official name is Enigma I (Roman number 1)
or Ch.11a.
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In 1926, the German Army - then called the Reichswehr - adopted the
lamp-based Enigma machine, or Glühlampenmaschine as it was called.
In early 1927, the first machine with an single-ended Steckerbrett
(plug board) was developed. It allowed each letter of the alphabet to be
transposed any other letter and can be seen as the equivalent of a
freely rewirable wheel that does not move.
Experiments showed that it was too easy to make mistakes with these cables.
Later that year the final version,
with an improved double-ended Steckerbrett was released.
It was based on the chassis of the Enigma D
and was given the internal designator Ch. 11a.
The first batch of machines (approx. 600 units) were delivered on 10 December 1927.
The Reichswehr called this machine the Enigma I.
All further German Army Enigma machines that were built before and
during the war, would be based on the Ch.11a.
The photograph above shows an Enigma machine in operation, and was
probably taken inside a radio van during WWII. The image was
scanned from an extremely small 27 x 38 mm original and was digitally
cleaned up and enhanced.
More wartime Enigma photographs
are available here...
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From 15 December 1938 onwards, each Enigma I was supplied with five
coding wheels instead of three, of which three were in the machine at any given time.
Each day, the operator would place the three chosen wheels in the machine in a particular order, as instructed by the
codebook.
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The remaining two (unused) wheels were stored in a small wooden box.
The image on the right shows a typical example of such a storage box with
the two wheels each held by a spindle.
Although other types of boxes are known to exist, this was the most
commonly used model.
Click the image for a closer look. As you can see, wheels I and V are
currentlty stored inside the wooden box, which means that the remaining wheels
(II, III and IV) are currently fitted inside the machine. With 3 out of 5
used, the number of possible wheel orders is 5 x 4 x 3 = 60.
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Each wheel has a single notch on its circumference. Whenever the wheel
reaches the position of the notch, a pawl is engaged. This pawl then moves
the next wheel (i.e. the wheel to the left of the current wheel) to be
moved by one position. This movement is called Enigma stepping.
The position of the notch is different on each wheel (see the wiring table below).
The downside of having only one notch on each wheel is that wheel movement
will be very regular and can therefore more easily be predicted.
Other machines, such as the Enigma G and the
Tirpitz (Enigma T) features multiple notches, but lacked
the addition of the Steckerbrett.
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In order to replace the battery or to change the daily key (Grundstellung)
of the Enigma, the top lid of the machine needs to be opened. A rigged
bolt with a red circle on top at either side of the lamp panel,
should be loosened in order to raise the hinged top lid and access the
interior.
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After raising the top lid, the wheel mechanism and the lamp panel are
exposed. The image on the right gives a good view at the drum.
At the right is the entry wheel, or Eintrittswalze (ETW).
Left of the ETW are the three cipher wheels
and at the far left is the reflector, or Umkehrwalze (UKW),
here marked with a red letter B.
Just visible at the bottom right is the
interior of the power switch.
It is activated by the knob
that is attached to the top lid.
Just above the power switch is a black box that holds the 4.5V
battery. The box is closed with a lid and a small lock.
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Below the wheels is the lamp panel.
It has 26 light blubs arranged in 3 rows. They are laid out in the
same order as the keyboard. Note that there are two additional
lamp sockets; one at either side of the middle row.
The extra socket at the right
is marked Lampenprüfung (lamp test). It can be used to
quickly test a lamp without typing on the
keyboard, simply by pressing it down.
The extra socket at the left
is marked Kabelprüfung (cable test).
If a lamp it fitted in this socket, it can be used to quickly test
a patch cable. On the Steckerbrett
an extra single-ended socket is present at
either end of the middle row of plugs. The extra sockets are marked
with a red dot. Now hold the tick pin of one plug to the leftmost socket,
and the thin pin of the other one to the socket at the right.
If the wire is OK, the cable-test lamp
should light up.
Then test the other wire.
Note that the light bulbs have an ordinary E10 fitting, but that
the glass bulb itself is flattened. Lamps of this time were commonly
used for bicycles and flash lights in those days.
Although it is possible to replace them by ordinary round bulbs,
this is not recommended as they will damage the
letter-film that is
mounted in the top lid. Never use ordinary light bulbs in an Enigma!
More about the lamps
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Sending a message with an Enigma machine involves the setting of
two cryptographic keys: (1) the daily key and (2) the
message key. Initially, the daily key was setup once a day
at midnight and was valid for 24 hours, hence its name.
Later in the war it was changed more frequently.
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Setting the daily key (Grundstellung) involved a number of steps.
First the top lid of the machine should be opened in order to access
the interior.
Next the UKW should be released
and shifted aside, so that the wheel-set
can be pressed together and
removed from the machine.
Now that the wheel-set is out of the machine,
the wheels are removed from the axle.
Next, the user would select the appropriate three wheels for the new
key and set the index ring as shown in the
codebook. The index ring can be released by lifting a
spring-loaded pin on its side.
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The wheels are now put back on the spindle in the order given in the
codebook, and then placed in the machine again. After locking the
UKW again, the top lid can be closed. The wheels should now be set
to the required start position,
which is visible through the windows in the top lid.
Now that the wheels are setup correctly, the plugboard (Steckerbrett)
needs to be configured as per codebook. This involves
removing all patch cables and re-inserting them as indicated.
The machine is now ready for use.
For each new message, the operator had to select a unique message
key, consisting of three randomly selected letters.
The exact method for setting the message was changed several times
during the war and is beyond the scope of this page.
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The double-ended Steckerbrett
had the advantage that it would swap the letters
in pairs and that the sockets had a built-in switch. If no swapping was
required, the cable could simply be left out. This greatly improved setup
time and reduced the chance of mistakes when setting the daily key.
Each Enigma comes with 12 cables: 10 to be used on the Steckerbrett and two
spares that are stored in the top lid of the case.
Each patch cable as a 2-pin plug at either side.
Each plug has a thick and a thin pin,
so that it can not be inserted
the wrong way around. The cable crosses the connection between
the plugs. The thick pin of one plug is connected to the other's thin one.
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The image on the right shows a double-ended plug with a thick and a thin pin.
Swapping the letters in pairs means that if A is transposed into Z, the
reverse is also true: Z is transposed into A. This is called
self-reciprocity.
Compared to the single-ended Steckerbrett, this reduces the total number of
possible wire combinations.
The same self-reciprocity was exploited by Gordon Welchman
when improving Turing's Bombe,
resulting in shorter Bombe-runs when breaking the Enigma's daily keys.
It effectively eliminated the Steckerbrett from the equasion.
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With 26 letters, and hence 26 sockets on the Steckerbrett, a maximum of 13
patch cables could be installed. Any number of cables between 0 and 13 was
possible and the maximum number of combinations would be reached when the
number of patch cables was different each day. In practice however, the
German operation procedure generally instructed the use of 10 cables.
More about the Steckerbrett
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Below is the wiring for each wheel, the ETW and all three known
UKWs. UKW-A was used before WWII [1]. UKW-B was the standard reflector
during the war and UKW-C was only temporarily used during the war.
The wiring of all 5 wheels is identical to the wiring of the
first 5 wheels of the Enigma M3 (used by
the Kriegsmarine) and the U-Boot Enigma M4.
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| Wheel
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
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Notch
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Turnover
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#
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| ETW
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
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| I
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EKMFLGDQVZNTOWYHXUSPAIBRCJ
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Y
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Q
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1
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| II
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AJDKSIRUXBLHWTMCQGZNPYFVOE
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M
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E
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1
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| III
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BDFHJLCPRTXVZNYEIWGAKMUSQO
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D
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V
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1
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| IV
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ESOVPZJAYQUIRHXLNFTGKDCMWB
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R
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J
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1
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| V
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VZBRGITYUPSDNHLXAWMJQOFECK
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H
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Z
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1
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| UKW-A1
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EJMZALYXVBWFCRQUONTSPIKHGD
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| UKW-B
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YRUHQSLDPXNGOKMIEBFZCWVJAT
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| UKW-C
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FVPJIAOYEDRZXWGCTKUQSBNMHL
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In 1945, immediately after WWII, some captured Enigma-I machines were
used by the the former Norwegian Police Security Service:
Overvaakingspolitiet.
They modified the wheel wiring
and the wiring of the Umkehrwalze (UKW, reflector).
The wiring of the Eintrittzwalze (ETW, entry wheel) and the position
of the turnover notches on the wheels were left unaltered.
A machine that is modified in this way, is often called
Norway Enigma or Norenigma as suggested by Frode Weierud
in 2001 in order to discriminate between the standard and the modified wiring [2].
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| Wheel
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
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Notch
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Turnover
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#
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| ETW
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
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| I
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WTOKASUYVRBXJHQCPZEFMDINLG
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Y
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Q
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1
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| II
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GJLPUBSWEMCTQVHXAOFZDRKYNI
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M
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E
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1
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| III
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JWFMHNBPUSDYTIXVZGRQLAOEKC
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D
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V
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1
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| IV
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ESOVPZJAYQUIRHXLNFTGKDCMWB
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R
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J
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1
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| V
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HEJXQOTZBVFDASCILWPGYNMURK
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H
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Z
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1
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| UKW
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MOWJYPUXNDSRAIBFVLKZGQCHET
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During WWII, the Germans made several attempts to
make the Enigma more secure.
In July 1944, the German Luftwaffe came up
with a smart device called Enigma Uhr, which was
introduced without any prior warning whatsoever.
It was attached to the side of an Enigma I and
connected directly to the Steckerbrett.
More information
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© Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Tuesday, 18 June 2013 - 17:16 CET
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