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Gretacoder 905
Portable electronic cipher machine

Gretacoder 905, also known as GC-905, was a portable electronic cipher machine, introduced in 1976 by Gretag AG in Regensdorf (Switzerland). The device is intended for off-line use and allows text messages to be encrypted and decrypted one character at a time, using just three buttons.

The device measures 197 x 120 x 40 mm and weights just 600 grams. At the top is the display that consists of 32 LEDs. Messages may contain letters, numbers and punctuation marks, but the ciphertext consists of the Latin letters A-Z only.

At the right is the MODE selector that allows the mode of operation to be selected: OFF, Encipher or Decipher. At the left is a large white field that is used to select the input-letter. Press at the left or right, to step through the available characters in both directions. Once the desired character is illuminated, press the small circular input key.
Gretacoder 905

This will cause the input letter to be encoded. The output letter will now start flashing on the display (as long as the input key is depressed). This method is used for enciphering, deciphering and for entering the Elementary Key. Full instruction can be found in one of the user manuals [B].

Gretacoder 905 was developed during the course of 1975 and was announced early in 1976. Like most Gretag cipher equipment, it was hated by the US National Security Agency (NSA), as it was unreadable to them. 1 The NSA had tried to get control over Gretag's algorithms, but to no avail.

In order to gain control of the crypto market, the American CIA and the German BND then decided that competitor Crypto AG — which they fully owned and controlled — should come up with a portable device that could compete with the Gretacoder 905 and could lure customers away.
Gretacoder 905 aside the HC-520 of Crypto AG (Hagelin)

The competing device — the Hagelin HC-520 — was developed by Siemens and was introduced in 1977. CIA and BND hoped that potential users would prefer the HC-520 over the Gretacoder [1]. The Gretacoder 905 was succeeded in 1981 by the Gretacoder 906, which had a full keyboard.

  1. In this context, unreadable means that the device could not be broken by the NSA. Also known as unfriendly or secure. In contrast: algorithms that can be broken, are known as readable or friendly.

Gretacoder 905 Gretacoder 905 compared to the size of a hand Selecting Decipher mode Pressing the NEXT button Gretacoder 905 aside the HC-520 of Crypto AG (Hagelin)
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Gretacoder 905
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Gretacoder 905 compared to the size of a hand
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Selecting Decipher mode
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Pressing the NEXT button
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Gretacoder 905 aside the HC-520 of Crypto AG (Hagelin)
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Although the Gretacoder 905 looks like a simple and perhaps crude device, it offered excellent security for its time. Operation was similar to that of the mechanical devices of the 1950s, like the Hagelin CX-52. The fact that a message are encrypted letter-by-letter was a disadvantage, but was at the same time a blessing in disguise. It forced its users to keep the messages short, and as a general rule of thumb, short messages are more difficult to break than longer ones.

Block diagram
The block diagram below shows how the GC-905 works. Note that it does not contain a micro­controller or any software whatsoever. Instead, it is fully built with discrete CMOS logic, and is controlled by various internal state machines. The block diagram is based on the description of its operation in US Patent 4,068,089. For a detailed description, please refer to that patent.

At the bottom are the controls: two push-buttons (left/right) for selecting the desired character, a push-button for entering the selection and stepping the state machine, and a rotary switch for selecting between encoding and decoding. Selecting the middle position switches the device off.

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The Gretacoder 905 is housed in a dark green plastic enclosure that consists of two case shells: the top half and the bottom halve. The two halves are held together by four M3 screws, that are accessible from the bottom. They might be protected by wax seals that have to be removed first.

Once the four screws have been removed, the bottom case shell can be removed, after which the interior is exposed. The complete assembly may now be removed from the upper case shell. It consists of three printed circuit boards (PCBs) and a holder for four 1.5V AA-size batteries.

The image on the right shows the complete assembly. The three PCBs are held together by red aluminum mounting stubs with C-clips in the four corners. They are inter­connected by means of short (fixed) ribbon cables. At the top is the display board. It holds the 32 red LEDs.
Stack of three PCBs

At the center is the control board, of which the left/right selector and the input button protrude a large cut-out in the display board. Also protruding the cut-out is the socketed PROM that holds the secret structure key. At the bottom of the stack is the actual encryption board that holds the shift-register logic. When the C-clips in the corners are removed, and the boards are separated, the wiring can be unfolded and the encryption board appears to be at the centre of the wiring.

Lower case shell removed Stack of three PCBs Character selector and input button, protruding the display board Three interlinked PCBs unfolded - solder side Three interlinked boards - component side Display board with 32 LEDs Encryption board Control board
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Lower case shell removed
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Stack of three PCBs
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Character selector and input button, protruding the display board
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Three interlinked PCBs unfolded - solder side
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Three interlinked boards - component side
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Display board with 32 LEDs
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Encryption board
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Control board

  • Alphabet
    Latin (other alphabets on request)
  • Cryptogram
    Letters only
  • Display
    32 LEDs
  • Modes
    Off, Cipher, Decipher
  • Period
    ≈ 108 (minimum)
  • Power
    4 × battery 1.5V (AA-size, type UM-3)
  • Temperature
    -20°C to +55°C (storage +40°C to +70°C)
  • Dimensions
    197 × 120 × 40 mm
  • Weight
    600 g
  • Structure
    PROM (user programmable read-only memory) ≈ 109
  • Elementary
    10 letters, determined by user = 2610 ≈ 1.4 · 1014
  • Auxiliary
    10 letters, automatically generated 1 = 2610 ≈ 1.4 · 1014
  1. At the start of each cryptogram.

  • Military-grade case
  • Power supply unit (PSU) 110/220V AC
  • Programmaing unit for PROM
Related patents
  1. The same patent is registered in Germany, France, United Kingdom, Japan, Netherlands, Norway and Sweden.

  1. Gretacoder 905 Operating Manual — draft
    Gretag Ltd., date unknown.

  2. Gretacoder 905 Operating Manual — final version
    Gretag Ltd., September 1976.

  3. Gretacoder 905, sales leaflet (English)
    Gretag Ltd., March 1976.

  4. Gretacoder 905, sales leaflet (German)
    Gretag Ltd., March 1976.

  5. Gretacoder 905, sales leaflet (French)
    Gretag Ltd., March 1976.

  6. Gretacoder 905, sales leaflet (Spanish)
    Gretag Ltd., March 1976.

  7. Wartungskonzeption Gretacoder 905
    Gretag AG, F. Christen, 26 January 1976.
  1. Crypto Museum, Operation RUBICON
    February 2020.
Further information
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Crypto Museum. Created: Wednesday 26 February 2020. Last changed: Tuesday, 21 April 2020 - 13:05 CET.
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