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Gretag SPG
Key production unit - under construction

Due to the way in which the human brain works, cryptographic message keys generated by human operators are never truely random. In many cases they are even predictable, e.g. when the name of a girl-friend is used. For this reason, Gretag develop the Schlüsselproduktionsgerät (SPG), an electro-mechnical device that produced random keys for the TC-53 and the TC-58.

The SPG is a rather small circular device, with a diameter of 12 cm, that is constructed in such a way that it could be built inside an existing TC-53 cipher machine. It was also suitable for the later TC-58 crypto extension for the KFF-58.

The TC-53 had a circular hole at the center of the front panel that was normally closed with a blank panel. After removing the black panel, the SPG could be mounted. Mounting it to the TC-58 was more cumbersome, as it needed a front panel with a suitable hole at the left. This hole is missing from most of the surviving TC-58 units.
  
Gretag Schlüsselproduktionsgerät (SPG)

Contrary to what its name suggests, the SPG is not a (pseudo) random number generator but merely a display unit. The actual key is generated by the crypto logic inside the cipher machine itself. In the TC-53 and TC-58 cipher machines this is called the Cryptogram Generator.

The pseudo-random data stream generated by the cipher machine consists of 14-bit data words. Five of these bits are used to feed the SPG [1]. Inside the SPG are 6 discs mounted on top of each other. Each disc has a series of notches and gaps around its circumference.

Five discs are driven by the 5 data bits from the cryptogram generator. The gaps on each of the discs are positioned in such a way that they represent the binary form of that bit. The SPG simply converts this 5-bit code into one of 32 positions (see below). The 6th disc is a shutter.
  
The letter 'C' shows up on the SPG

The SPG was considered a highly classified device and was only supplied to the Swiss Army. TC-53 and TC-58 devices that were used by other countries, such as Austria, did not have an SPG. Please note that, although the SPG is part of a TC-53 or TC-58 and is controlled from these devices, it is not part of the encryption algorithm. It is just used to generate random message keys before encrypting a text. TC-53 and TC-58 machines are perfectly usable without the SPG.

Gretag Schlüsselproduktionsgerät (SPG) Front view of the SPG Rear view of the SPG Close-up of the electromagnets A Siemens diode mounted at the center 8-wire connection to the host device 8-pin socket for connection to the host device

TC-53 Operation
When encrypting, each new message should be encoded with a different key. On the Enigma machine, this key consisted of the daily key (Grundstellung) and a message key that was randomly picked by the operator. The operator then sent the message key, entrypted with the daily key, to the other end. Both parties then set their machine to the message key, after which the rest of the message was sent. Altough in theory this method works relatively well, in practice it appeared to be difficult to persuade operators to pick truely random message keys.

For the Gretag machines, a similar system was used, but in order to ensure that the message key was not, say, the operator's girl-friend's name, a time-consuming procedure involving the SPG was developed. For the generation of the message key, the internal key generator (also known as the cryptogramm generator) of the TC-53 (or TC-58) was used. In order to generate a message key, we need 12 letters, each of which represents the start position of a wheel.

First, the operator of the initiating station produced a 12-digit number consisting of randomly picked digits 1-9 (0 was not allowed), for example 235684166719. This number was then sent to the station at the other end in clear. Both stations then switched to Key Production Mode and the wheels were set to the daily key.

The image on the right shows the Gretag TC-53 cipher machine with the (optional) SPG mounted at the center of the lower half of the front panel.
  
Gretag TC-53 with SPC mounted at the center. Click for more information about the TC-53.

Next, the individual digits of the number that was transmitted in clear, were used to determine the number of steps that the internal key generator had to make before producing a letter for the message key. So, from the basic position, the key generator was stepped twice (the first digit) and the operator wrote down the letter that was shown on the SPG (e.g. 'B'). The key generator was then stepped three times (the second digit) before the next letter was written down (e.g. 'L'), and so on, until all twelve letters of the message key had been written down, e.g.:

BLCHQIOXATKZ

Both ends would now set their wheels to the new starting positions, as indicated by the message key, and the initiating party was able to type the actual message. If the key at the other end was set correctly, the clear text would appear automatically at the receiving end. According to former users of the SPG in the Swiss Army, the generation of a message key by means of the SPG could take up to 10 minutes (!) which is why pre-generated key lists were often used instead.

External SPG
As the operational procedure described above was too cumbersome for some customers, Gretag developed an external SPG that could be connected to the expansion connector of the TC-53 (Zusatz). It was released in 1957 and was offered to customers as a field upgrade. The advantage of the external SPG unit, was that it would automatically make the required number of steps.

The upgrade required the existing SPG to be removed from the TC-53 and mounted inside the new device that was called Zusatzgerät für halbautomatische Schlüsselproduktion (add-on device for semi-automatic key production). The hole in the TC-53 was then closed with a circular black panel; the same one as used on the TC-53 machines that were exported (e.g. to Austria) [2].

TC-58 Operation
The SPG could also be added to the later TC-58 (KFF-58) cipher machine. It was mounted at the left side of the front panel of the cipher unit and required a special front panel (with a large hole). On machines that were sold outside Switzerland, this large hole was not present. Operation of the SPG was identical to the external SPG for the TC-53, as described above. The machine would automatically make the required number of steps.

Interior
The SPG is a self-contained unit that is connected to the host machine by means of a rather stiff grey cable with an 8-way socket at the end. This socket mates with a plug that is present inside the host device. Inside the SPG are six electromagnets (solenoids) that are mounted in a circle.

Each of the solenoids controls a disc with teeth and gaps. On five discs the teeth and gaps are spaced differently, in such a way that each one represents one bit of a 5-bit digital counter.

Each disc has 32 teeth and 32 gaps and has two positions: 0 (solenoid OFF) and 1 (solenoid ON). When in position 1, the disc is rotated 1/64th of a circle. The discs are all spaced in such a way that the teeth represent the binary value of the corresponding bit. By stacking the five discs on top of each other, there will always be exactly one position where the gaps coincide.
  
Close-up of the electromagnets

As a result, one of the white indicator lines on the alphabet disc at the bottom will be visible, acting like a pointer to one of the letters. As each disc can be in two positions, we have 25 or 32 possibilities. So the mechanism can be seen as a so-called 5-to-32 decoder. The image below explains how it is constructed. The upper drawing shows how the various discs are stacked.

Interior of the SPG Arms of the electromagnets Disassembled SPG Bare SPG unit without the discs Removed discs Disc number 1 (bit 0) Stack of discs Cable variant for mounting inside the TC-53



The second drawing above shows an exploded view of the counter. For clarity, each disc has been given its own colour. To ensure that the white lines are not visible when the counter is not in use, an extra shutter disc is mounted on top of the stack. It is controlled by the 6th electromagnet.


The tables above and below show how the teeth and gaps are spaced and what the purpose of the mask and the shutter is. In the above table, all five electromagnets are disengaged (binary code 00000), representing position 0. As the shutter is closed, nothing is visible in this state.


If the shutter is now opened (i.e. the 6th electromagnet is activated), the gap at position 0 becomes visible, revealing part of the white circle on the alphabet disc at the bottom. This white spot acts as an indicator to the corresponding letter on the alphabet ring: in this case the 'A'.


Now assume that the key generated by the number generator inside the TC-53 is 11, which is represented by the binary code 01011. When the corresponding electromagnets are activated, the position of the discs will be as shown in the table above and the gaps at position 11 will coincide. As a result the letter 'L', will be indicated. The function of the mask (i.e. the bottom line of each table) is to indicate the active positions of the discs (the 0s represent the white lines).

Key generation
From the above it has become clear that the SPG is only a display device and not a (random) key generator. The actual key is derived from the internal key generator (also known as cryptogramm generator) of the host cipher machine (the TC-53 or the TC-58). That generator is based on a 12-wheel driven mechanism that produces 26 digital bits. 12 of these bits control the stepping of the 12 wheels, whilst the remaining 14 bits are used for the encryption and decryption of text. 1

Of these 14 key-bits, five are used to drive the SPG.

  1. On the TC-53 and TC-58, each letter is represented by a 14-bit digital code as used with the ETK teleprinters, rather than the more common 5-bit baudot code that is used by most other teleprinters.

References
  1. Gretag AG, Telekryptogerät TC-53, Fuktionsbeschreibung
    Nr. 500-2. Functional description (German). 1953.

  2. Dr. Edgar Gretener AG, Zusatzgerät für halbautomat. Schlüsselproduktion
    Additional device for key generation (German). 5 October 1957.
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