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OMI Criptograph
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 wartime OMI Alpha and contains 5 cipher wheels, a settable stator and a settable reflector.

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.
OMI Criptograph. Photograph courtesy NSA [1].

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.

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.

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 reversible (reciproke). 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.

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 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:

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.

Physical wiring
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:

           R atmiwjkxyzuvsrqponlhgfedcb
           L abcdefghlnopqrsvuzyxkjwimt

Logical wiring
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.

  1. 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.
  2. 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'.

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.

Circuit diagram
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 marked a few years later.

 Circuit diagram of the Cryptograph-CR

  1. Herbert Avram, Ottico Meccanica Italiana Cryptograph-Wiring
    NSA-064. Reference ID: A56952. Informal No. 16. 17 June 1954.
    Declassified by NSA on 20 May 2014.
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Crypto Museum. Created: Sunday 20 December 2015. Last changed: Saturday, 24 February 2018 - 14:07 CET.
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