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Rotor OMI CR → ← Alpha
Electromechanical cipher machine
- wanted item
Criptograph was a wheel-based
electromechanical cipher machine,
developed and produced by
OMI in Rome (Italy) just after WWII, around 1954.
It was intended for use by the Italian Armed Forces, but was also offered
to foreign customers [1]. The machine is the successor of the WWII
OMI Alpha,
and contains 5 electrical cipher wheels, a settable stator and a settable
reflector.
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As far as we know, there are no surviving examples of this machine.
The image on the right was taken from a recently declassified NSA document,
in which the various features of the machine are described and discussed [1].
The document was issued in 1954, probably after the NSA had
obtained a machine for evaluation.
The image on the right was taken from the NSA report and shows the layout
of the machine, which is very similar to that of its successor,
the Cryptograph-CR, that has the cipher wheels in the
same position and is also motor driven.
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The machine is very similar to the
German Enigma,
with its cipher maze, or drum,
consisting of an entry disc,
5 cipher wheels (rotors), a stator and a
reflector.
It features simple stepping of the rotors, which means that each
cipher wheel has only one notch, or cam, that causes stepping of the
adjacent wheel, in the same way as the odometer of a car.
According to the report, the manufacturer has suggested
that it would be possible to construct rotors with multiple cams.
Unlike the Enigma however, the Criptograph is driven by an electromotor,
and prints its output directly onto a paper strip, making it much faster
in operation than the Enigma.
The design is clearly based on the wartime OMI Alpha,
which had a simpler construction but similar features.
In the late 1950s, the Criptograph was succeeded by the somewhat improved
Cryptograph-CR.
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The diagram below rougly shows the position of the various controls
and features of the OMI Criptograph. At the front is the keyboard with the
26 letters of the Latin alphabet organised in the QZERTY order.
Note however that the letter 'W' is used as the spacebar. Immediately behind
the keyboard is the printer, which consists of a double print head that
is driven by a commutator at its rear. It prints directly onto a paper
strip, that is fed in from a supply drawer at the right.
The cipher wheels are placed to the left of the centre of the machine,
with the longitudinal axis of the drum running from front to rear.
The cipher wheels are driven by a set of cogwheels that are part of
the wheel coupling at the centre. The entire mechanism is driven by
an electromotor at the rear right. It drives the main axle that runs
from front to rear, just behind the printer.
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OMI Criptograph in the NCM collection
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During the COVID-19 pandemic of 2020 and 2021, the
National Cryptologic Museum (NCM)
at the NSA headquarters in Fort Meade (USA)
was closed to the public, whilst the building was renovated.
As there were no public activities, it gave the curators and
registrars the ability to go through the entire collection to see
whether any objects needed maintenance or special attention.
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And it was on one of these occasions, in June 2020, that the museum registrar
stumbled on the machine shown in the image on the right.
A complete and original OMI Criptograph that was manufactured
around 1954, in a blue hamerite enclosure with a green hamerite
keyboard.
At present, no further information about this particular machine is
available, but it will no doubt be investigated further. New information will be
added as and when it becomes available.
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The cipher wheels are located inside the machine, left of the centre,
with the longitudinal axis running from the front of the machine towards
the rear. Each wheel has 26 contacts at either side and, hence, has 26
possible positions, each of which is identified by one of the letters
of the Latin alphabet (A-Z). The letters are printed in such a way that
they are readable from the position of the operator. There are 7 wheels
in total, 5 of which are mounted on a removable axle. The full wheelset
is known as the 'drum'.
Now look at the drum from the left side of the machine:
The drum consists of five moving cipher wheels, a stator and a reflector.
The cipher wheels are moved when a message is entered on the keyboard.
The stator does not move, but can be set to any of its 26 positions.
The reflector doesn't move either, but like the stator it can be set
to any of its 26 positions. Only they 5 cipher wheels can be removed.
They are mounted on a spindle.
The operation of the drum and the flow of the electric current is very
similar to that of the Enigma.
The wiring from the keyboard is connected
to the entry disc
at the far right, which consists of 26 flat-faced
contacts. From there, the current enters one of the spring-loaded
contacts of the rightmost wheel. The wiring inside the wheel
transposes the current to one of its flat-faced contacts at the left side.
This way, the current passes through all 5 cipher wheels and the Stator (S),
until it arrives at the Reflector (R)
on the left. The current is then returned.
The simplified schematic diagram above shows how the current flows
from the entry wheel, through the cipher wheels, the stator, the
reflector and back. The entry path is shown in red, whilst the return
path is blue. In the example, the letter 'A' is encoded into 'D'.
Due to the fact that a reflector is used, the path is reciprocal (reversible).
This means that, at the same settings and positions
of the wheels, the letter 'D' would be encoded into 'A', just like
on the Enigma.
It also means that the machine has the same weakness as Enigma, in that
a letter can never be encoded into itself. In other words: if 'A' is
pressed, it can become any letter, but never the 'A'.
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The 5 cipher wheels (i.e. the rightmost five wheels when looking
at the drum from the left side of the machine) are mounted on an
axle that can be removed by releasing the drum-locking levers at
either end of the drum. Before doing this however, the
gear coupling
behind the wheels (again when looking from the left side) should
be disengaged first.
The spindle can now be removed.
Before removing the wheels from the spindle, the stop at one
of its ends has to be removed using a special tool (if supplied).
If this tool is not present, a large screwdriver that precisely fits
the slit can be used as an alternative. The wheels can now safely
be removed from the spindle.
Each cipher wheel consists of three main parts: a metal frame,
a red wiring core and a black wiring core, as shown in the image
above. Although each frame has a unique number (I thru V), they
are all identical and have a wheel-turnover notch at the letter 'A'.
Each wheel is fitted with two unique wiring cores: a black one
and a red one. The black one is always fitted at the right (or
the front when seeing it from the operator's perspective) with the
spring-loaded contacts facing outwards. Likewise, the red core is
always at the left with its flat-faced contacts facing outwards.
The frame is no more than a die cast holder for the two cores,
each of which can be inserted in 26 different positions. The red core
is always inserted from the left, in such a way that the 26
spring-loaded contacts mate with the 26 holes in the centre part of
frame. The core is then screwed to the frame using using the fold-out
clip at the centre. In the same way, the black core can be inserted
into the right side of the frame in 26 ways, and is secured
in place with the clip.
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When the top lid of the case is in place, the 7 wheels protrude
the top surface of the lid so that their position can be viewed
and altered.
Each of the wheels has the 26 letters of the alphabet
printed around its circumference, in such a way they can be read
by the operator when operating the keyboard. The wheel stepping
mechanism shows great resemblance to that of the
Zählwerk Enigma,
in that it is driven by cogwheels. This allows corrections to be
made, simply by turning the advance/reverse knob just behind the
keyboard. Each wheel has only one turnover notch.
When typing a character on the keyboard, the rightmost wheel
(i.e. the wheel closest to the front of the machine) moves
counter clockwise (from the operator's perspecitive) to the
next position. This means that after 'A' the 'B' shall be visible.
As each frame has only one turnover notch, this means that the
second wheel will make a single step after a full revolution of
the first one.
Note that the turnover notch, or cam, is mounted to the wheel
aside the letter 'A', but will only cause the next wheel to step
when the letter 'T' is visible in the window at the top of the machine.
The Stator (S) and Reflector (R) do not move. To illustrate the stepping
of the five cipher wheels, we show three successive steps of the fast wheel,
as seen from the operator's position:
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Each machine was supplied with at least 5 black cores and 5 red cores,
but additional cores were often provided to increase
the possible key space. Each core is marked with a white number that is
engraved between the A and B contacts. As an example, the drawing below
shows both sides of red core number 4 and black core number 5.
Each combination of a red and a black core forms a complete cipher
wheel and each core can be inserted in the frame in 26 different
positions.
In the original description, black and red
cores are referred to
as front and rear cores respectively
[1].
At both sides of each core, the 26 letters of the Latin alphabet are
used as an index. They are embossed in the bakelite surface and are
in a mixed order.
Starting with the letter 'A' at the white index mark,
the alphabet runs clockwise around each face of the cores,
in the following order:
ATMIWJKXYZUVSRQPONLHGFEDCB
ABCDEFGHLNOPQRSVUZYXKJWIMT
The upper line shows the order of the input contacts (i.e. the spring-loaded
contacts), whilst the lower line gives the order of the output contacts
(i.e. the flat-faced contacts), assuming that the cores were wired straight
through. Viewing the core from the side, shows us the following layout:
The machine at the NSA, was supplied with 16
cores: 8 red cores for the left side of the wheel (in the document referred
to as the rear cores) and 8 black cores for the right side of the wheel
(front cores). The front cores have been given odd numbers.
The rear cores have even numbers. Of the 16 cores,
10 are in the machine at any time (5 red and 5 black ones),
subject to the key settings.
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The contacts of the wiring cores of the cipher wheels of the Criptograph
are marked with a mixed alphabet, as explained above. This makes it very
difficult to describe the machine's wiring in a uniform manner. The fact
that the index runs clockwise at both surfaces of the cores, make it even
worse. The entry disc, or end plate, is wired in the same
mixed order, with the 'C' contact at the top, alhough entry discs with
the regular alphabetic sequence were also known to exist [1].
In order to avoid confusion when describing the wheel wiring, the NSA
converted the mixed index on the cores, to the regular order
of the alphabet, running clockwise on the front surface (right) and
counterclockwise on the rear surface (left), using the table below.
In this table, lower case letters are used to describe the physical index
that is embossed on the wiring cores, whilst the capitals represent
the transposed (logical) order. A straight-through wired core looks like this:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
R atmiwjkxyzuvsrqponlhgfedcb
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L abcdefghlnopqrsvuzyxkjwimt
ABCDEFGHIJKLMNOPQRSTUVWXYZ
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The table below gives the logical wiring of the cores, after
converting the mixed alphabet index on the wiring cores to the regular
alphabetic order, in the manner described above.
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Wheel
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Colour
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
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Remark
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1
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black
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QUOEVGYXLNIKZRJSFMHPTBAWCD
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Front
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2
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red
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KDVZEHYTRPIGNQBASXJLOFCWUM
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Rear
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3
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black
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SFJNITODCAEKULRVHGBXWYMPQZ
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Front
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4
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red
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LJCOXPTSEQVGRYHIUFKAZBMWSN
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Rear
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5
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black
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EHILSKYZMJCRXWQUPNFDAGOBVT
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Front
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6
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red
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MJWVNQOCUHBDFKGZYXLEIRAPTS
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Rear
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7
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black
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QPJRNOYLUDTMABCSWZGIHXVEFK
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Front
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8
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red
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HVEYSGRABPXMWINKJLFDZCTQUO
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Rear
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9
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black
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LKEHPNGBDMTOCWUYRAJZFQVSIX
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Front
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10
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red
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BNPRSFTVYUDGCQOEMILHAZKXWJ
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Rear
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11
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black
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LEPKHQIOCDTWXAZUVNBGMRFJSY
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Front
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12
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red
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EURAHITDYFMBXJGWNCLKZPVQOS
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Rear
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13
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black
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BIKWXYAZVQCFGSLNPEHJUTMDRO
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Front
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14
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red
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MBGFSRCQYLXWVEHDUNPOZAIJKT
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Rear
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15
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black
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HELIQKJCDRNMPBTWVAZYFGXSOU
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Front
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16
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red
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INBAZYNHVLETDFGCSOPQWXRKUJ
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Rear
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S
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CSEHXJLVNKWGDZPYOBUFQTMRAI
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Stator 1
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R
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RSLVOKXPMQFCIZEHJABUTDYGWN
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Reflector 2
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The core of the stator can be fitted into the ring in two ways:
with the 'A' at the front of the core lined up with the '6' on
the ring, or with the 'A' lined up with '19' (i.e. rotated by 180°).
The wiring shown here is with the 'A' at '19', which is how the machine
was delivered at the NSA.
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In the report [1] the wiring of the reflector is given relative
to the references on the ring (01-26), but it does not state the
position of the 'A'. We have therefore assumed that the 'A' is at '01'.
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At present we have no high-resolution images of the machine's interior,
but a relatively good black-and-white image of the bottom side is presented
in the NSA report [1]. It reveals that the wiring is very similar to
that of the later machines.
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Although the original circuit diagram of the Criptograph is not available
to us, and was not part of the NSA report either [1], it seems resonable to
assume that it was nearly identical to that of its successor,
the Cryptograph-CR,
which appeared on the market a few years later.
➤ Circuit diagram of the Cryptograph-CR
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© Crypto Museum. Created: Sunday 20 December 2015. Last changed: Wednesday, 21 February 2024 - 22:48 CET.
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![OMI Criptograph. Photograph courtesy NSA [1].](img/omi_criptograph_full.jpg)
