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Rotor OMI Criptograph →
Electromechanical cipher machine
- wanted item
Cryptograph-Alpha, or Alpha, is a wheel-based
electromechanical cipher machine,
developed and produced in secrecy by
OMI in Rome (Italy) around 1939, at the start
of WWII. It was intended for use by the Italian Army
(Regio Ersetico), the Air Force (Regia Aeronautica) and the Navy (Regia Marina).
It is similar to the German Enigma
and has 5 cipher wheels, including a moving reflector.
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The machine features irregular stepping of the cipher wheels,
controlled by stepping pins, or notches, at the circumerference of
each wheel, similar to the stepping of the
Zählwerk Enigma.
The difference with the Enigma however, is that the Alpha has one more
cipher wheel and that it is motor-driven, making it faster in operation.
Furthermore, it prints its output directly onto a paper strip, allowing
the machine to be operated by a single person. The keyboard has 26 keys,
representing the 26 letters of the Latin alphabet, of which the letter 'W'
is used as the spacebar.
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The image above shows a typical Alpha cipher machine in Naval
grey colours. The machine was developed by
OMI in Rome, an opto-mechanical
company that had specialised in cartography and the production of
photogrammetic equipment during the interbellum. 1
The outbreak of WWII led to an unexpected interruption of the production at
OMI-Nistri and sister company
SARA-Nistri,
and the company had a hard time surviving. Luckily they were
given secret orders by the Italian Government for the development of
a high-end cipher machine for the Italian Armed Forces.
The OMI Alpha is very similar to the Zählwerk Enigma,
but is more advanced. Nevertheless
it is not widely known, probably due to the fact
that little information about these machines is available in the public
domain. Only a small number of machines have survived.
Once WWII had ended, OMI gradually
resumed their usual business again, but also developed several successors
to the OMI Alpha, again for the Italian Armed Forces,
starting with the OMI Criptograph in 1954.
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The interbellum is the period between WWI and WWII.
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The diagram below shows the various controls and features of the OMI
Alpha. The mechanism is built on a raised chassis, with the keyboard
at the front. The keys are in the QZERTY order, with the 'W' being used
at the space bar. An extra paper advance key is present at the bottom right.
At the front left is the built-in printer that produces its output on a
paper strip that is fed in from the right. The five cipher wheels are located
at the right, immediately behind the paper supply, and protrude the top
cover of the machine, so that they can be set without opening the machine.
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The five cipher wheels are located in the right half of the machine and
protrude the top cover, protected by a transparent hinged flap, so that
the user can set the message key without opening the machine. In order to
change the order of the wheels, the top cover has to be removed first.
The top cover is locked with a physical lock that is located at the top
centre of the front panel, indicating that only officers with
the proper security clearance were allowed to set the daily key.
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The image on the right shows the cipher wheels after
the top cover has been removed.
Each wheel has 26 spring-loaded contacts at its right side and 26 flat-faced
contacts at its left side. The spring-loaded contacts of the rightmost wheel,
mate with the 26 flat-faced contact of a contact plate, or entry disc, 1
at the far right.
The wheels are mounted onto a spindle that is entered into the machine
from the right side. It is protected by a hinged lid and by yet another
physical lock, again indicating that the order of the wheels could only
be changed by an officer.
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The leftmost wheel is the reflector that moves during operation,
in the same way as the leftmost wheel on the
Zählwerk Enigma.
In fact, the design of the Alpha is so similar to that of the Enigma,
that it seems reasonable to assume that the designers had detailed
knowledge about them.
The reflector 2 only has 26 contacts at its right side and returns the
current into the cipher wheels.
The simplified circuit diagram above shows the flow of the current through
the cipher wheels when pressing a key. In this example, the letter 'A' is
pressed and the current is passed to contact 'A' of the entry disc (ED).
From there, the current is scrambled by the cipher wheels
(in the order IV, III, II, I), until it leaves the
leftmost cipher wheel (I) on the left. The reflector (R), which contains 13
wires, returns the current through the cipher wheels
(in the order I, II, III and IV),
until it leaves the drum via the entry disc again. In the example, the
letter 'A' is converted into the letter 'D'.
The four cipher wheels (I, II, III and IV) are interchangeable and can be
put on the spindle in 24 different orders (4 x 3 x 2). The reflector is
different from the other wheels as it has contacts on one side only.
It is fixed in place and has a cogwheel attached to its body (left) that
is advanced by one step on each key press. All five wheels have a so-called
ring setting 3 that allows the internal wiring maze to be rotated
with respect to the body of the wheel and, hence, the stepping notches.
The ring setting can be altered with a small tab, even when the wheels are installed.
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The entry disc is similar to the Eintrittswalze
(ETW) on the Enigma.
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The reflector is similar to the Umkehrwalze (UKW)
on the Enigma.
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This is similar to the ring setting, or Ringstellung, on Enigma.
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Although the wheels stepping mechanism of the OMI Alpha is nearly identical
to that of the Zählwerk Enigma, it has to be noted
that, compared to Enigma, the wheel stepping order works from left to right,
rather than from right to left. In other words: the reflector is the fast rotor.
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On each key-press, the leftmost wheel (i.e. the reflector) makes a single step,
during which the top of the wheel moves towards the rear of the machine. This means
that if the letter 'A' was visible at the top of the wheel, the letter 'B' will be
visible once the key has been pressed.
Each wheel has a full set of 26 pins at its left side, and a limited number of
pins at its right. The limited number of pins on the right side of the reflector
are adjacent to the full set of 26 pins of the cipher wheel to its right. They are
coupled by a small cogwheel behind the wheels.
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Whenever a pin is present at the right side of a wheel, it will drive the cogwheel,
which in turn causes the adjacent wheel to make a single step. This way, the
reflector accasionally causes the first cipher wheel to step, the first cipher wheel
occasionally causes the second wheel to step, and so on. The cipher wheel at
the far right does not drive anything and is the slowest moving one.
The image above shows the complete drum with the entry disc on the right and the
reflector on the left. At the top are the small cogwheels through which wheel
stepping takes place. The advantage of this mechanism is not only that it features
irregular wheel stepping, but also that it can be used to make corrections.
The upper axle (marked correction) extends through the right side of the machine
and allows the wheels to be stepped forward as well as backward.
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The wiring of the cipher wheels of this machine is currently unknown.
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The interior of the OMI Alpha
can be access easily by removing the top
cover, using a physical key to open the lock at the centre top of the
front panel. After removing the top cover, the rather simple construction
of the machine is revealed. The cipher wheels
are located at the front right.
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The mechanism is driven by an electromotor that is placed immediately
behind the cipher wheels. It drives a small gear box to its left which
in turn drives the printer, at the left of the frontpanel,
via a mechanical coupling.
The timing of the printer is controlled by
an electric commutator that is mounted in between the front panel and
the chassis. It is only visible from the top.
The image on the right shows the interior of the OMI Alpha, as it is
seen from the rear left of the machine. The motor is at the left
and the gear box is at the center. The printer is at the right.
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The printer mechanism is rather simple and consists of a handful of
cogwheels, pawls and levers. Its timing is controlled by an
electric commutator
that is driven by the output of the cipher drum. The commutator
is just visible through a rectangular hole in the top of the frame
(top right).
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© Crypto Museum. Created: Sunday 20 December 2015. Last changed: Thursday, 01 July 2021 - 16:20 CET.
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