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U-Boot Enigma
This Enigma machine was developed during WWII, exclusively for the U-Boot
division of the Kriegsmarine (Germany Navy).
It played a vital role in the Battle of the Atlantic
and was introduced completely unexpected on 2 February 1942,
as the successor to the Enigma M3.
It caused great upset with the allied codebreakers at
Bletchley Park,
who called it the Shark-key.
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Like the other naval machines, the design is based on the
Enigma I, that was already in use
by the Wehrmacht and Luftwaffe.
It has three moving code wheels, a fixed
reflector (UKW) and a Steckerbrett
(plug board).
It was supplied with 8 different coding wheels, (I to VIII),
3 of which were in the machine at any given time. The wiring of wheels I to V
was identical to those of the Enigma I.
Unlike the Army, the Navy choose to have letters (A-Z) on the
circumfere of each wheel, rather than numbers (01-26).
The wiring of each wheel is given in the table below.
Besides the 3 extra wheels to choose from, an extra wheel
was added to the M4, to the left of the 3 moving wheels. This extra
wheel forms an additional stage in the ciphering process.
The extra wheel is not moved during encypherment and can not
be exchanged with the other three wheels. When the extra wheel
is placed in the A-position, the machine
is compatible with
the 3-wheel
Enigma I
and the Enigma M3.
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Below is the simplified circuit diagram of the M4. It is similar
to the circuit diagram of the Enigma I,
but has an extra coding wheel at the left (4).
If you don't understand how the Enigma works, you might want to
read about the working principle first.
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Simplified circuit diagram of the Enigma M4
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The M4 is in fact a modified 3-wheel machine. The width of the
UKW has been halved and the remaining space is taken up by
the 4th wheel, which has spring-loaded contacts at either side.
As it is thinner than the other wheels, it is sometimes called
the thin wheel.
It is also referred to as the Griechenwalze (Greek wheel) as it was
identified with the Greek letter Beta or Gamma.
The official name for the 4th wheel was Zusatswalze (additional
wheel).
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The Enigma M4 was supplied with 8 coding wheels, but only 3 of them
would be in the machine at any time. Each day, the operator would
pick 3 wheels from the available 8 and place them in the machine
in a particular order, as instructed by the
codebook.
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The remaining 5 (unused) wheels were stored in a special
wooden box. The image on the right shows such a box with the
unused wheels in it.
The box has space for 7 wheels but only 5 positions were used.
The remaining positions were probably meant for a future
extra wheels or to store additional gadgets, such as UKW-D.
More pictures of this box below.
Click any of the images to enlarge. Note that the serial number
of the wheels is present on the box' ID-plate and is also engraved in the lock.
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Wheels I to V each have one turnover notch, but the three extra
(Naval) wheels each have two notches. As you can see in the
images below, these two notches are positioned opposite each
other.
The construction of the Naval wheels is different from other
Enigma wheels. Rather than using a spring-loaded pin to set the
Ringstellung (common on all other wheels), the user has to press two
pawls simultaneously (see the rightmost image below).
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Additional wheel (Zusatzwalze)
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The Enigma M4 is the only machine that has an extra wheel to
the left of the 3 normal coding wheels. This wheel was called
the Zusatzwalze (extra wheel) or
Griechenwalze (Greek wheel) as it was identified with the
Greek letter Beta.
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This is the main reason why the M4 is often called the
4-wheel Enigma machine.
The Zusatzwalze could however not be picked from the total
set of 8 wheels, as it was constructed differently:
It has spring-loaded contacts at either side.
To accomodate this, the UKW has 26 contact pads.
Furthermore, the wheel is not driven by the other wheels.
In other words: it is never moved during encipherment.
In fact, together with the UKW, it just forms a selector between 26
different UKWs.
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Initially, only one Zusatzwalze (Beta) was supplied together
with a thin version of UKW B.
The UKW and the Zusatzwalze were wired in such a way that
when the Zusatzwalze was put in the A-position the combination
(Beta + UKW B) was compatible with UKW B on the
Enigma I
and M3.
Later in the war, a new set was supplied (Gamma + UKW C) but
this was not an overwelming success, so they soon revered to UKW B.
If both Beta and Gamma were present with a machine, another
wooden box was used to store the unused UKW and Zusatzwalze.
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Please note that althoug the Zustzwalze can be set to any position,
it does not move during the ciphering process. In other words:
it can't be moved by the wheel to the right of it. This could be
considered a weakness as it limits the possible number of permutations
of the entire system. In fact, the combination UKW-Zusatzwalze can
be regarded as a selection between 26 different UKWs.
Another weakness of the wheelturnover mechanism is that the wheels
move regularly. Only after the rightmost wheel has completed a
full revolution, does it cause the next wheel to make a single step.
As a result, the 2nd wheel (from the right) will only make one step
every 26 characters and the 3rd wheel will hardly ever move.
This makes the machine more predictable and easier to break.
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The three extra wheels (VI, VII and VIII) have two notches each,
which causes a more frequent wheel turnover. This was done as the 3rd wheel
(from the right) hardly ever moved during the ciphering process.
Note, however, that the wheels have 2 notches which is a relative prime
of 26 and that the notches are positioned oposite each other (see
the image on the right).
In practice
this will halve the cipher period, which was yet another weakness.
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Furthermore, cipher instructions commanded the use of at least one
of the extra 3 Naval wheels every day, and that such a wheel could
not be used in the same position on two successive days.
This was known by the code breakers and reduced the number of possible
permutations.
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During WWII, some attempts were made to make Enigma traffic more
secure. In January 1944, a field-rewirable reflector, called UKW-D,
was introduced by the Luftwaffe (German Air Force).
It is little known that an UKW-D was also developed for the Kriegsmarine
(Navy).
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The image on the right shows an UKW-D with a serial number starting
with the letter M. It was developed especially for the 4-wheel M4 machine.
When in use, it would replace both the reflector (UKW) and the extra wheel
(Zusatzwalze). The letter D is engraved in the body of UKW-D at such a
position that it can be seen through the leftmost window of the Enigma's cover.
 More information
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The Enigma M4 has a lock in the top cover that can be used to prevent the wheel
settings from being changed. This was probably done to allow only an officer
to change the wheel settings. The Enigma M3 even has two
of such locks.
As far as we know, locks were only present on the Enigma machines used by
the Kriegsmarine (Navy).
It is quite possible that the lock was only used in a limited number of
situations, as only very few keys have been found.
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The image on the right shows an extremely rare key for the Enigma M4.
Note that the machine's serial number is engraved on the key. M4 keys
generally do not match the lock of another M4, but in our case we were lucky;
the key shown here fits the lock of our M4 perfectly, despite the fact that
it has a different serial number.
If no original key is present, it is also possible to have a replica key made
from a so-called blank, even whithout removing and disassembling the
lock. For this, one would need to call the help of a skilled lock-picker.
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The rightmost four images below show a replica key that was made in 2008 by
Barry Wels
of the Dutch lockpickers organisation Toool.
He used a technique called impressioning and it took him just under
10 minutes to create a smoothly operating key.
More about this on Barry's Weblog.
Another example of an original Enigma M4 key is presented in
image #4 below.
It belongs to Dutch collector Arthur Bauer, who found it in a small paper
bag that was stored in between the Enigma machine and its wooden box.
One day, when cleaning the machine, the bag simply fell out.
His M4 is now on permanent display at the
Churchill War Rooms in London.
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Below is the wiring for each wheel, the ETW and all known
UKWs. UKW-B was the standard reflector throughout the war and UKW-C
was only temporarily used during the war.
The wiring of the first 5 wheels is identical to the wiring of the
5 wheels of the Enigma I used by the Wehrmacht
and Luftwaffe.
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| Wheel |
ABCDEFGHIJKLMNOPQRSTUVWXYZ |
Notch |
Turnover |
# |
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| ETW |
ABCDEFGHIJKLMNOPQRSTUVWXYZ |
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| I |
EKMFLGDQVZNTOWYHXUSPAIBRCJ |
Y |
Q |
1 |
| II |
AJDKSIRUXBLHWTMCQGZNPYFVOE |
M |
E |
1 |
| III |
BDFHJLCPRTXVZNYEIWGAKMUSQO |
D |
V |
1 |
| IV |
ESOVPZJAYQUIRHXLNFTGKDCMWB |
R |
J |
1 |
| V |
VZBRGITYUPSDNHLXAWMJQOFECK |
H |
Z |
1 |
| VI |
JPGVOUMFYQBENHZRDKASXLICTW |
HU |
ZM |
2 |
| VII |
NZJHGRCXMYSWBOUFAIVLPEKQDT |
HU |
ZM |
2 |
| VIII |
FKQHTLXOCBJSPDZRAMEWNIUYGV |
HU |
ZM |
2 |
| Beta |
LEYJVCNIXWPBQMDRTAKZGFUHOS |
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| Gamma |
FSOKANUERHMBTIYCWLQPZXVGJD |
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| UKW-B |
ENKQAUYWJICOPBLMDXZVFTHRGS |
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| UKW-C |
RDOBJNTKVEHMLFCWZAXGYIPSUQ |
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The three extra wheels (VI, VII and VIII) have two notches each,
which causes a more frequent wheel turnover, but also introduces
another weakness (see above under Wheel turnover).
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Differences between M4 and other Enigma machines
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Looking at an Enigma M4, a number of physical differences with other
Enigma machines can be observed.
The most obvious differences are listed here.
Photographic evidence of each feature below the list.
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- Position of the lock
The lock, that keeps the wooden box closed, is mounted in
the top lid and not in the bottom part as on all other machines.
- Carrying handle
The M4 has a metal carrying handle that is recessed in the
wooden case, whereas most other machine have a leather carrying strip.
- Removable top lid
The top lid does not have the metal support brackets
that keep it in a stable position when opened. In fact, the top
lid has to rest against, say, a wall or has to be removed completely
when operating the M4. Removal of the top lid is easy as the
M4 only has simple hinges at the rear.
- Metal brackets
Each M4 also has two handles, one at either side of the main body,
that allows the machine to be lifted out of a bay in the U-Boot
radio operator's table.
- Removable lamp panel
The cover of the lamp panel is removable whereas on all other
Enigma machines it is part of the wheel cover. The reason for
this is that on the M4 it was possible to remove all lamps and
replace them with a printer (e.g. Schreibmax).
- Zusatzwalze (extra wheel)
The M4 is the only machine that has an additional (4th) wheel
that is not interchangeable with the other wheels. For this
reason the machine is often called a 4-wheel Enigma.
See also the description under the heading Zusatzwalze
above.
Check also the comparison between the M4 and
a standard Service Enigma.
- Different plug length on the Steckerbrett
The pins of the plugs (Steckern) on the plugboard are about
4 mm longer that on the Enigma I. Using the plugs of an M4
on the Steckerbrett of an Enigma I will cause permanent
damage. The reason for this difference is unknown. For the
same reason, the Enigma Uhr can't be used
on an Enigma M4.
- Power socket
First of all the battery inside the M4 has a different size.
It is about half the size of the battery used in the other Enigma
machines. Furthermore is has a connector that can be connected
directly to the 4 Volt line of a vessel or U-Boot.
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