CR Mk.2 →
Electromechanical cipher machine
- wanted item
Cryptograph-CR is a wheel-based
electromechanical cipher machine, developed
and produced by OMI in Rome
(Italy) in the late 1950s, as the successor to
the OMI Criptograph
1 . It was intended for use by the
Italian Armed forces (in particular the MMI 2 )
and probably some foreign users.
The image on the right shows a typical example of a Cryptograph-CR
as it was on display at the
HAM Radio in 2013
. The machine is typically
housed in a green hammerite enclosure and is stored in a wooden green
At the front is a 27-button keyboard on which the letter 'W' is missing.
This was done as the 'W' isn't used in the Italian language. It is replaced
by the space bar. The arrow-key at the bottom right is used for
continuous paper transport. A resettable counter is visible through the top
lid. It shows the number of entered characters.
The Cryptograph-CR is similar in operation to the
German Enigma, in that it
has a set of electromechanical cipher wheels with 26 contacts
at either side, an entry disc (called Eintrittzwalze or ETW on Enigma)
and a reflector (Umkehrwalze or UKW).
Unlike Enigma however, the OMI Cryptograph-CR
has a built-in printer that produces the output on two paper strips at the
front of the machine, just behind the keyboard. One strip shows the plaintext;
the other one the ciphertext.
The paper strip is supplied from the right side of the machine,
passes the printer at the front center, and leaves the machine on the left.
The empty paper reels are stored in a drawer at the right side of the machine.
In the early 1960s, some of the Cryptograph-CR machines received an upgrade
in which the (partly) mechanical keyboard was replaced by a fully electrical
one, with 26 relays controlling the input and output signals. At the same time,
the machine was painted grey hammerite. To avoid confusion, we've named that
machine the Cryptograph-CR Mark II
Note the spelling of the name Criptograph
rather than Cryptograph.
MMI = Marina Militare Italiana (Italian Navy).
The diagram below shows the various controls and features of an OMI
Cryptograph CR of which the top lid has been removed. The cipher wheels
are at the left, with its axle running from front to rear and the entry disc
closest to the operator. The stator and the reflector are at the rear.
The cipher wheels are driven by a series of cogwheels that are located at
the center of the machine, just below the wheel coupling. An electric
coupling (i.e. a solenoid) ensures that the entire mechanism makes one full
revolution on each key-press. At the front of the machine is a double
printer. The two print heads are identical and are mounted on the same axle.
A double commutator ensures that each of the print hammers is released
at exactly the right moment.
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 will
transpose 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'.
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
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.
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
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
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:
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 3 and black core number 4.
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
In the original description, black and red
cores are referred to
as front and rear cores respectively
At both sides of the core, the alphabet (A-Z) is embossed in the
bakelite surface as an index. Note that in both cases, the alphabet
runs clockwise, which means that it should only be used as a reference
when inserting the core into the frame. It should not be used when describing
the wheel wiring or the cipher algorithm. The letter 'A' is at the same
position on both sides.
Note that on the earlier version of the machine
(i.e. the Criptograph),
the alphabet index letters on the sides of the cores were in a transposed order,
probably to add an extra layer of obscurity. Apparently this transposition was
abandoned on the later Cryptograph-CR. Furthermore, with the earlier
odd numbers were used for the front cores (black) and even
numbers for the rear cores (red).
With the later machines, this no longer seems to be the case, but there
are indications that numbers were not duplicated (e.g. if there is
a red core 3, there is no black 3).
The wiring of the cores, the stator and the reflector of the OMI
Cryptograph-CR featured above, is currently unknown, but we were able
to take down the wring of an upgraded version of the machine, the
OMI Cryptograph-CR Mk II,
which might be similar or even identical.
➤ Wiring of the Cryptograph-CR Mk II
The drawing below show the circuit diagram of the Cryptograph-CR.
The mechanical parts are at the right and are shown in blue. The machine
has three individual power supply units (PSU). PSU 1 provides the +250V
DC voltage for the Coupling Solenoid (V1).
PSU 2 provides the voltage for the cipher drum (V2)
and PSU 3 provides the HT and LT voltages
for the valve-based solenoid drivers.
After switching the machine ON, the motor start running, but the
other mechanical parts are in rest. If a key is pressed, it is locked
mechanically whilst the Coupling Solenoid is activated (VC).
The coupling sets the main shaft in motion.
During this motion, the pressed key is mechanically locked in place.
It is released again after one full revolution of the main shaft.
Below each of the 26 letter keys on the keyboard is a 3-pole double-throw
(3PDT) switch. The first pole of this switch is used
to drive the coupling solenoid (VC) that sets the
main shaft in motion.
The second pole is used for the actual encryption of the letter and is
similar to a keyboard switch on the Enigma cipher machine. Below is an
example of encoding a letter.
When pressing the 'A', voltage (V2) is passed to the
corresponding contact of the Entry Disc (ED).
Once the letter is encoded, the returned signal (in this example
the letter 'B') is sent to the Ciphertext Printer (C2).
As the machine also has a Plaintext Printer,
the third pole of the switch is used to send the original character
directly to the Plaintext Printer (C1).
The Cryptograph-CR has two built-in paper strip printers at the front of
the machine. The rearmost one is used for showing the plaintext, whilst
the one at the front is used to print the ciphertext. The two printheads
are identical and are driven by the same shaft. Two individual commutators
(C1 and C2) that are also driven by this shaft, determine
the precise moment to activate each of the printer hammers. The hammer
pushes the paper strip against the print head.
Note that commutator and print head rotate in synchronism.
As the current that is needed for the hammer solenoid is too high for
the cipher wheels, a valve-based theratron amplifier is used.
The CIPHER/DECIPHER switch to the right of the keyboard (CLEAR/CIPHER)
simply swaps the two amplifier inputs, so that the plaintext also appears
on the rearmost printer when decoding.
Declassified by NSA on 20 May 2014. EO 13526.
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© Crypto Museum. Created: Sunday 20 December 2015. Last changed: Thursday, 01 July 2021 - 21:25 CET.