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OMI Cryptograph-CR
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 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 [1]. The machine is typically housed in a green hammerite enclosure and is stored in a wooden green transport case.

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 or CR-2.

  1. Note the spelling of the name Criptograph rather than Cryptograph.

OMI Cryptograph-CR
OMI Cryptograph-CR with top lid removed
OMI Cryptograph-CR interior
Close-up of the commutators
Cipher wheels
Input/out commutator connectors
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OMI Cryptograph-CR
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OMI Cryptograph-CR with top lid removed
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OMI Cryptograph-CR interior
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Close-up of the commutators
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Cipher wheels
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Input/out commutator connectors

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'.

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.

5 cipher wheels on a spindle
The drum after removing the cipher wheels
Wheel stepping gear
Gear engaged
Gear disengaged
The drum, looking into the entry disc
The drum, looking into the stator
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 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 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 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

Detailed images of the interior of this machine are currently unavailable. To get some idea of the mechanical construction, please refer to the interior of the upgraded OMI Cryptograph-CR Mk II which is mechanically identical.

 Interior of the Cryptograph-CR Mk II

Circuit Diagram
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.

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.

  1. Crypto Museum, Ham Radio 2013
    OMI Cryptograph-CR courtesy John Alexander.

  2. Herbert Avram, Ottico Meccanica Italiana Cryptograph-Wiring
    NSA-064. Reference ID: A56952. Informal No. 16. 17 June 1954. 1
  1. Declassified by NSA on 20 May 2014. EO 13526.

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Crypto Museum. Created: Sunday 20 December 2015. Last changed: Wednesday, 21 February 2024 - 22:59 CET.
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