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Zählwerk Enigma
Around 1927, Enigma manufacturer Chiffriermaschinen AG
started the development of a series of new machines, all derived
from the commercial Enigma D.
For the Reichswehr (the predecessor of the Wehrmacht),
they developed the Enigma I and for various
other customers (both civil and military) the
Enigma K family was introduced.
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At the same time, development started on an improved - more advanced -
machine that was known as:
Glühlampen-Chiffriermaschine "ENIGMA" mit Zählwerk und zwangläufiger
Kupplung der Chiffrierwalzen.
Translated: Lamp-Enigma with counter and coupled cipher wheels.
It has a cog-wheel driven wheel-turnover mechanism that features
irregular stepping, making it cryptographically much stronger than
the Enigma D.
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The machine is often referred to as the Enigma G because later
production series had serial numbers starting with the letter 'G'.
This is, however, not the case for all Zählwerk machines;
the earlier machines had serial numbers starting with the letter 'A'.
The code breakers at Bletchley Park called
it the 11-15-17 machine, after the number of notches on each wheel.
The machine is also called the Abwehr Enigma as it was
used to some degree by the German Secret Service, the Abwehr,
during WWII. It should be noted however that this was not the only
cipher machine used by the Abwehr, and that the machine was also used
by civil and military customers in several countries, including
Hungary and The Netherlands.
In practice, the machine was mostly called Zählwerksmaschine or Zählwerk Enigma
(Counter Enigma) which is the name we will use on this page.
The Zählwerk Enigma was built in several flavours and was improved
a number of times.
Furthermore, a smaller variant (model G31) was introduced in 1931.
We should like to thank Frode Weierud in Switzerland for his
help in sorting out the history and the various models of the
Zählwerk Enigma. His research in the German
Archives has proved invaluable on more than one occasion.
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The Zählwerk Enigma is clearly based on the design of the Enigma D,
albeit with some additional features and improvements. Most of these
features are described in the two German patents
DE534947
[1]
and DE579555
[2]
.
The most striking difference with the other Enigma models is the way
in which the wheels are moved. In the Enigma D (and also in
the Enigma I used by the German Army), the wheels are moved by
means of pawls, rachets and notches. As a result, these machines can
only step forward.
In the Zählwerk Enigma, however, the wheels are moved by a gearbox-like
mechanism with cog-wheels. Furthermore, the number of notches on each
wheel has been increased drastically, and is different for each wheel.
The longest cipher period is generated when different relative prime
numbers are used for the number of notches on each wheel. And this is
exactly what they did in the Zählwerk Enigma.
Another difference with the Enigma D is that the UKW (reflector)
can not only be set to any of 26 positions, it is also moved by the other
wheels during encipherment. The three coding wheels are mounted on a
spindle, just like in most other Enigma machines, whilst the UKW is
fitted permanently.
Each cipher wheel has a full cog-wheel with 52 teeth attached to its
right side. On the left of the wheel is another cog wheel with the same
spacing, but with a number of teeth missing. The presence of a set of tooth
is equivalent to a notch on an ordinary wheel.
When the wheels are engaged, they are coupled by means of 4 small cogwheels
with teeth of alternating length.
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Ordinary Enigma wheels (left) and the wheels of a Zählwerk Enigma (right)
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As a result, the entire mechanism can be stepped forward and backward,
without losing the relation between the position of the wheels. A crank
can be inserted into a hole in the body of the machine, to wind the
machanism to the desired position. This was used to correct mistakes,
but could also act as part of the (message) key.
The wheels of the standard Zählwerk machine have the same diameter
as the wheels of other Enigma machines, such as the Enigma D.
With the G31 model, however, smaller wheels were supplied as illustrated
in the drawing above. In order to accomodate the spring-loaded contacts,
they are arranged in a zig-zag pattern and the contact pads at the other
side have an oval shape.
Most machines were supplied with just 3 wheels that could be mounted on
the spindle in 6 different orders. These wheels (I, II and III) had
17, 15 and 11 notches respectively. The positions of these notches are
identical for all recovered machines, regardless of their wiring and
regardless of the customer. Some machines were supplied with more
wheels (e.g. 5 wheels were supplied to Hungary).
It is likely that this machine was initially intended for commercial use,
as the wiring of the UKW and the coding wheels of some recovered machines
appeared to be identical to the wiring of the commercial Enigma D.
In some cases, the wheels were rewired by the customer, but in many
cases the wiring of the UKW was left unchanged.
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Versions of the Zählwerk Enigma
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- Early Zählwerk Enigma (1927)
This machine was built before the patents of its design were
actually filed and should probably be regared as a pre-production
series. The counter was placed to the right of the cipher wheels
and the wheels each had a single turnover notch, which means that it
featured regular stepping.
- Zählwerk Enigma, model A28 (1928)
This machine is identical to the drawings in German patents
DE534947
and DE579555.
The cipher wheels are coupled with a gearbox-like mechanism and
each wheel has a different number of turnover notches, all
relative primes of 26. The latter would greatly increase the
cipher period. On this model, the counter was located to the left
of the cipher wheels.
 Detailed description
User Manual of the A28 (courtesy FRA Sweden)
- Zählwerk Enigma, model G31 (1931)
In 1931 a smaller version of the Zählwerk Enigma was introduced.
These machines not only had a smaller body, but were also equipped
with smaller cipher wheels.
At the same time the battery compartment
was removed and the counter was moved to the right again.
These machines all had serial numbers starting with the letter 'G',
which is why they are often called Enigma G.
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For a long time, we wondered why the Abwehr Enigma
(model G31 from 1931) looks so different from the other models,
and how it was developed. Patents DE534947
and DE579555 (1928) were known of course,
but the machine described in those patents, is very different from
the later model G31.
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All that changed when, in 2007, we discovered an original Zählwerk Enigma
that matches the patent descriptions of 1928. The machine, with serial
number A865, was probably built in 1928 and was sold to a commercial customer
shortly afterwards. It was called: model A28.
It is extremely well built, using only the highest quality components and
is clearly based on the chassis of the earlier Enigma D (1926).
In fact, it looks just like an Enigma D at first glance. It has the
same dimensions, it shows four coding wheels and has shiny metal parts.
Like in the Enigma D, the rightmost three wheels are the actual coding
wheels, whilst the leftmost one is the reflector (UKW).
Unlike the Enigma D however, in which the UKW can be set to any of 26
positions, the UKW of this machine is moved during encipherment.
Furthermore is has a counter (Zählwerk) to the left of the wheels
and it is the first machine that features a cog-wheel driving wheel-turnover
mechanism.
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At the top left of the machine is a metal lever with two settings:
AUS/EIN (OFF/ON). This is used to disengage the cog-wheels, so that
the wheels can be set to any start position.
The counter (to the left of the coding wheels) is incremented on each
key-press and does not have a reset button.
A crank, that is stored inside the top lid, can be inserted into a small
hole to the right of the coding wheels, close to the power switch.
The crank can be used to wind the entire cog-wheel mechanism back or forth.
This was used to correct mistakes, but could also be used as part of the
coding process (i.e. the message key).
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Under the bonnet
When the cover is opened, the beautiful cog-wheel driven wheel-turnover
mechanism is revealed.
The wheels have the same size as those of the earlier Enigma models,
but the turn-over mechanism is completely different.
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The picture on the right gives a clear view of the coding wheels,
the movable UKW (on the left), the counter and the hole for the
crank. This image shows a striking resemblance with the drawings
in patent DE579555
[2]
.
It is the first machine that features the cog-wheel mechanism,
a movable UKW and multiple turnover notches. It is clearly
the design on which the later model G31 was based, and we can
therefore regard this machine as the ancestor of the Enigma G
(Abwehr Enigma).
The missing link is found.
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More detailed pictures of the interior of this machines can be
found below.
When the wheels are removed, we get a good view of the
Eintrittswalze (entry wheel, ETW).
Surrounding the ETW is the main driving cog-wheel. Behind the
UKW is the stepping lever that moves a saw-teeth wheel on
every key-press.
Coupled with the ETW are several smaller cog-wheels that allow
the crank to be used to wind the entire mechanism back and forth.
The smaller cog-wheels also drive a thin axle that, in turn,
drives the counter at the left. Just behind the counter is the UKW
that can not be removed from the machine. To the left of the UKW
is the usual rounded lever, that is used to disengage the UKW,
so that it can be moved to the left, allowing removal of the
coding wheels.
Behind the UKW is the cog-wheel coupling lever. When it is in
the EIN position (ON), all wheels are coupled and can be moved
during encipherment. When the lever is in the AUS position (OFF),
the coupling is disengaged and the wheels can be moved freely,
allowing the message key to be set.
When the machine is closed, the coupling lever protrudes a hole
in the metal cover.
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The wheels
The wheels of the Zählwerk Enigma have the same size as those of the
earlier machines. The outer diameter is approx. 100 mm (4 inches) and
the hole for the spindle is also the same. The positions of the 26 contact
pads and the spring-loaded contacts on the other side of the wheel,
are identical.
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The image on the right shows the left sides of the wheels of a Zählwerk Enigma
(left) and an Enigma I (right) side by side.
The rightmost wheel shows a single triangular gap at the top,
which carries on the wheel on its left by means of pawls and rachets.
In contrast, the wheel of the Zählwerk machine (left) has a series of notches
around its circumfere. These notches can be regarded as the a large cog-wheel
with 52 teeth, of which a number of teeth are missing.
Each wheel has a different numbe of notches.
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The cog-wheels of all wheels are coupled via a spindle at the rear, containing
three small cog-wheels with an alternating series of long and short teeth.
The coupling can be engaged and disengaged by using the coupling lever at the
top left of the machine.
In order to increase the cipher period of the machine, each wheel has a
different number of notches, all being relative primes of 26. Furthermore,
there is no common factor between the numbers.
Wheels I, II and III have 17, 15 and 11 notches respectively.
Apparently, the position of the notches was never changed, as all known
Zählwerk machines have these notches at the same position, including the
later G31 models.
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Ordinary Enigma wheels (left) and the wheels of a Zählwerk Enigma (right)
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When this machine was found, the UKW and all cipher wheels had
the same wiring as the (commercial) Enigma D. This suggests that the
Zählwerk Enigma was initially intended as a commercial machine.
The machine (and also the later G31) was also sold to the military
and to some foreign customers.
Some of the latter changed the wiring of the cipher wheels, but in many
cases the wiring of the UKW was left unaltered.
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| Wheel |
ABCDEFGHIJKLMNOPQRSTUVWXYZ |
Notch |
Turnover |
# |
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| ETW |
QWERTZUIOASDFGHJKPYXCVBNML |
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| I |
LPGSZMHAEOQKVXRFYBUTNICJDW |
ACDEHIJKMNOQSTWXY |
SUVWZABCEFGIKLOPQ |
17 |
| II |
SLVGBTFXJQOHEWIRZYAMKPCNDU |
ABDGHIKLNOPSUVY |
STVYZACDFGHKMNQ |
15 |
| III |
CJGDPSHKTURAWZXFMYNQOBVLIE |
CEFIMNPSUVZ |
UWXAEFHKMNR |
11 |
| UKW |
IMETCGFRAYSQBZXWLHKDVUPOJN |
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Model G31: The Abwehr Enigma
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The Zählwerk Enigma model G31 is sometimes called The Abwehr Enigma,
as it was used, to some extent, by the German Seret Service the Abwehr.
Please note that the Abwehr also used other machines, and that the Abwehr was
not the sole user of this Enigma model.
A good example of a Zählwerk Enigma used by the Abwehr is the G-312
[3]
that is on public display in the museum at Bletchley Park.
It is this machine that was stolen from the museum in 2000.
Luckily, Enigma historian David Hamer had just completed a
detailed description of the machine,
which was quickly spread on the internet.
As a result, the machine could not be traded easily and was returned
several months later to BBC reporter Jeremy Paxman.
The case, and hence the wooden box, of the G31 are somewhat smaller
that the case of an Enigma D
or an Enigma I.
This was probably done to make the machine more portable.
The outer dimensions of the wooden box are 25x27x16.5 cm.
As the case is smaller, there is no room for a battery pack and the
machine therefore has to rely on an external power source.
The wheels of the Enigma model G31 are also smaller than those of
all other Enigma machines. The largest diameter is approx. 85 mm (3.5 inches),
whereas the normal wheels are 100 mm (4 inches). In order to accomodate
the 26 spring loaded contacts, they are arranged in a zig-zag pattern.
The contact pads at the other side of the wheel have the shape of
a tear-drop.
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The wheels of the initial Zählwerk machine (left) and the Enigma model G31 (right)
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The Abwehr used different wiring than the other customers of this machine.
In order to keep the wiring secret, they ordered unwired wheels
from the manufacturer. Furthermore, different departments of the Abwehr
used differently wired wheels, and it seems likely that, during the course
of the war, the wiring was changed several times.
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The table below shows the wiring of the G-312. Although the machine is
believed to have been used by the German Abwehr, it is the only
one every found with this wiring. Different wirings were used for different
sections of the Abwehr, and also for different radio nets. It is also
possible that some machines were rewired during their lifetime.
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| Wheel |
ABCDEFGHIJKLMNOPQRSTUVWXYZ |
Notch |
Turnover |
# |
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| ETW |
QWERTZUIOASDFGHJKPYXCVBNML |
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| I |
DMTWSILRUYQNKFEJCAZBPGXOHV |
ACDEHIJKMNOQSTWXY |
SUVWZABCEFGIKLOPQ |
17 |
| II |
HQZGPJTMOBLNCIFDYAWVEUSRKX |
ABDGHIKLNOPSUVY |
STVYZACDFGHKMNQ |
15 |
| III |
UQNTLSZFMREHDPXKIBVYGJCWOA |
CEFIMNPSUVZ |
UWXAEFHKMNR |
11 |
| UKW |
RULQMZJSYGOCETKWDAHNBXPVIF |
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In March 1945, just before the end of WWII, the Argentine police arrested the
German spy Johann Siegfried Becker. In his posession was an Enigma model G31
with serial number G-260. Two months later, they handed the machine over to the
Americans
[4]
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As Becker was believed to work for the German Secret Service,
the Abwehr, it is most likely that the G-260 was wired for Abwehr
communication.
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| Wheel |
ABCDEFGHIJKLMNOPQRSTUVWXYZ |
Notch |
Turnover |
# |
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| ETW |
QWERTZUIOASDFGHJKPYXCVBNML |
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| I |
RCSPBLKQAUMHWYTIFZVGOJNEXD |
ACDEHIJKMNOQSTWXY |
SUVWZABCEFGIKLOPQ |
17 |
| II |
WCMIBVPJXAROSGNDLZKEYHUFQT |
ABDGHIKLNOPSUVY |
STVYZACDFGHKMNQ |
15 |
| III |
FVDHZELSQMAXOKYIWPGCBUJTNR |
CEFIMNPSUVZ |
UWXAEFHKMNR |
11 |
| UKW |
IMETCGFRAYSQBZXWLHKDVUPOJN |
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G-111: A rare version of Zählwerk Enigma G31
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In May 2009, a very rare variant of the Zählwerk Enigma model G31 turned
up at auction house Hermann Historica in Munich (Germany). The machine
with serial number G111 would be up for auction in October 2009.
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It appeared to be a G31 model from the first production batch (the numbers
started with G101) that had a hitherto unseen connector at its left side.
In July 2009, we were given the opportunity to research this machine in
detail. We have since written a paper about this rare machine.
More information
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