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OMI Cryptograph-CR Mk II
Electromechanical cipher machine - wanted item

Cryptograph-CR Mark II, or CR-2, is an electromechanical cipher machine, produced by OMI in Rome (Italy) in the early 1960s, as an upgraded version of the earlier the OMI Cryptograph-CR 1 . It was intended for use by the Italian Armed forces, in particular by the Italian Navy (MMI). 4

The machine was initially sold as a (green) Cryptograph-CR, and was given a mid-life up­grade after several years of service. The original (partly) mechanical keyboard was removed and replaced by a fully electric one, of which the keys are placed closer together. The spacebar was dropped and the letter 'W' was added, so that the machine now supports the full A-Z alphabet.

The unused holes of the keyboard are covered by a new panel that is mounted over it. At the same time, the case of the machine was re­painted in grey hammerite as shown here. 2
OMI Cryptograph-CR Mark II (courtesy Günter Hütter [1]

The two switches that enabled plaintext and ciphertext printing were also dropped, and their holes were covered. As a result, the machine now always prints both plaintext and ciphertext. Inside the machine, parts of the Power Supply Unit (PSU) were modernised and 26 electric relays were added, each of which is connected to one of the switches on the keyboard. In order to accomodate these relays, the paper supply drawer at the bottom right has been given up as well.

The machine 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 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 machine shown here was used by the Military Operational Intelligence Service 3 (SIOC) of the Italian Navy 4 (MMI), as indicated by an extra name tag at the rear. The name tag also suggests that this machine is based on the 1954 model. It is believed that the Cryptograph-CR and the Mk II variant were the last OMI cipher machines to be made. There are not many surviving examples.

  1. After the modification, the machine kept its original model name 'Cryptograph-CR'. The suffix 'Mark II' has been added by ourselves to identify the modified or upgraded version.
  2. At some places on the machine's body, the old green hammerite paint is still partly visible.
  3. SIOC = Servizio Informazione Operazione di Combattimento.
  4. MMI = Marina Militare Italiana.

OMI Cryptograph-CR Mark II
Start position of the cipher wheels
Letter counter
Gear coupling
Printer window
Serial number
Italian Navy identification plate
The diagram below shows the various controls and features of the OMI Cryptograph-CR Mark II. The upper section of the interior is largely identical to that of the original Cryptograph-CR, but the keyboard section is completely different. It now carries all 26 letters of the Latin alphabet (A-Z) plus a paper-advance key. Furthermore, the keys are placed closer together, making it easier to operate. The print-enable switches at the right (plaintext and ciphertext) have been removed.

Controls and features of the OMI Cryptograph-CR Mk II

The motor compartment is at the rear right. Note that the motor is missing from the machine shown above. The paper strip printer is placed just above/behind the keyboard. The cipher wheels are at the left, with a mechanical (cogwheel) coupling to their right. The cipher wheels can be released by unlocking two levers: one at either end of the axle.

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'.

Cipher wheels
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.

Cipher wheels and gear coupling (seen from the right)
Entry disc and drum locking lever
Rear drum locking lever
Gear engaged
Gear disengaged
The drum after removing the cipher wheels
Wheel stepping gear
The drum, looking into the entry disc
The drum, looking into the stator
5 cipher wheels on a spindle
5 cipher wheels on a spindle
Left side of a cipher wheel
Right side of a cipher wheel
Black core removed
Red core removed
Wheel body, red core and black core
Wheel stepping
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:

Wiring cores
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 positions. In the original description, black and red cores are referred to as front and rear cores respectively [2].

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 algorithm. Only the letters 'A' and 'N' are 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 Criptograph, 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 following cores were found in the Cryptograph-CR Mk II that is featured on this page [1]:

Physical wiring
The table below gives the wiring of the OMI Cryptograph-CR Mk II that is featured on this page. Please note that the wiring is given relative to the A-Z index that is present at both sides of each wheel. Please note that the A-Z index runs clockwise at both sides (when viewing the contacts).


The above table gives the physical wiring, taking into account that the A-Z index is different on each side of the wheel. Only the letters 'A' and 'N' are at the same position. In order to use the wiring for a proper description of the algorithm, the following transposition should be applied:

Index on the output side of the disc (left) ABCDEFGHIJKLMNOPQRSTUVWXYZ
Corrected index for the output side AZYXWVUTSRQPONMLKJIHGFEDCB
Logical wiring
The logical wiring of the wheels is obtained when applying the above transposition table to the output side of each disc. For this we assume the right side (i.e. the spring-loaded contacts) to be the input, and the left side (i.e. the flat-faced contacts) to be the output. The table below gives the logical wiring for each wheel, relative to the A-Z index on the input side (right).

The interior of the machine can be accessed from both sides: the top and the bottom. At the top are the mechanical parts, such as the cipher wheels, the printer, the main driving axle, the motor and the commutator. At the bottom are the electric parts, like the power supply and the relays.

The main driving axle is placed at the center of the machine, running from rear to front. It has an electric coupling and is driven by an electro­motor that is placed at the rear right. Note that the motor is missing from the machine shown here, but that it has since been replaced as part of the restoration process. The printer, also visible in the image, is placed at the front end of the driving axle, just behind the keyboard.

The printer features two alphabets: one for the ciphertext and one for the plaintext. A paper strip runs right-to-left below the print heads.
Main driving axle and commutator seen from the rear

The two print heads are coupled, so that they always rotate synchronously. Each print head has a hammer, mounted below it, that ensures that the correct letter is printed onto the paper strip. These two print hammers are driven asynchronously, under control of a double commutator.

The bottom side of the machine contains the electric components, such as parts of the power supply unit (PSU) and the mains transformers.

This side is completely different from the earlier Cryptograph-CR. The paper tray has been given up and its space has been used to accomodate a set of 26 electric relays, organised as two rows of 13 relays each. Each relay is driven by one of the keys on the keyboard and offers 4 sets of contacts. In addition, a fairly large master relay is mounted towards the rear. It drives the main coupling and is controlled by the other relays.
Close-up of the relays

The reason for adding the 26 relays to the design was probably to lower the current through the keyboard contacts, especially when driving the coupling. Furthermore it simplifies the keyboard. For a detailed description of the working of the machine, please refer to the circuit diagram.

Main driving axle and commutator seen from the rear
Commutator (seen from the rear)
The two commutators disconnected
Bottom view
26 relays, main relay and 200V DC PSU
Order of the relays
Close-up of the relays
Main relay
Mains entry socket and 200V DC PSU
Removable capacitor
Cipher wheels and gear coupling (seen from the right)
Entry disc and drum locking lever
Seen from the rear
Circuit Diagram
The drawing below show the circuit diagram of the Cryptograph-CR Mk II. 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 to drive the relays (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 turning the machine ON, the motor starts running, but the other mechanical parts are in rest. As soon as a key is pressed, the corresponding relay (Re) is activated which in turn activates the main relay (VC) that drives a solenoid driven coupling. The coupling sets the main shaft in motion. A Hold Switch, activated by a tacho disc on the main shaft, ensures that the activated relay is held (VH) and that the coupling is released after one full revolution of the main shaft.

For each of the 26 letter keys on the keyboard, a Siemens 4-pole double-throw (4PDT) relay is present. The first switch of this relay is used as a hold contact. It is connected to the Hold Switch (VH) to ensure that the relay remains activated whilst the main shaft is in motion. The second contact is used to drive the coupling solenoid (VC) that sets the main shaft in motion.

Example of how the letter 'A' is encoded into 'B'

The third switch is used for the actual encryption of the letter and is similar to a keyboard switch on the Enigma cipher machine. It supplies a voltage (V2) to the corresponding contact of the Entry Disc (ED). Once the letter is encoded, the returned signal is sent to the Ciphertext Printer (C2). As the machine also has a Plaintext Printer, the 4th relay switch is used to drive that directly (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.

Simplified operation of a printer

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.

June 2016

The machine featured on this page belongs to Austrian collector Günter Hütter [1], and has now been fully restored so that it can be demonstrated again. A suitable motor with appropriate gear and the correct speed has been mounted in the empty space at the rear right, as shown below.

Although the speed of the motor is not crytical, it has to be sufficiently slow to allow the various parts of the cipher mechanism and the printer to move smoothly. The replacement motor has a speed of 1300 RPM, which appears to be perfect.

The selenium rectifier bridge and the electrolytic capacitor of the 200V DC power supply turned out to be broken and had to be replaced. The coil of one of the individual alphabet relays was also broken but could not be repaired. Luckily, an identical replacement relay of the same type, spec and build date was found in the junk box.
Replacement motor mounted in the CR Mk II

Once the broken parts were replaced, the contacts of the rotor basket were cleaned and the gear box was properly greased again, it was time to put to the motor in action for the first time after many years. The mechanism worked straight away and after adjusting the commutator (to meet the speed of the new motor) a text was enciphered and deciphered correctly. OMI is alive again.

Replacement motor mounted in the CR Mk II
Rear view of the motor
Commutator, main gear and motor
Motor gear
Replacement motor specification
Repaired power supply
Close-up of the relays
Cipher text and clear text
  1. Günter Hütter, Cryptograph-CR Mk II
    OMI Cryptograph-CR Mk II courtesy Günter Hütter. Photographs: Crypto Museum.
    Austria, June 2015 and June 2016.
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© Crypto Museum. Created: Sunday 20 December 2015. Last changed: Thursday, 01 July 2021 - 21:21 CET.
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