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Slidex
Maxtrix-based coding of fixed messages

    A new and more detailed description of Slidex is currently under construction. Once the new page is ready, it will replace this one.  sneak preview
Slidex was a simple manual cipher system based on a matrix of fixed words and frequently used phrases. It was introduced by the British Army during WWII and was simultanously used by the Russians. Since then, it has been used in many different variations. It is not very secure and is suitable only for short-term tactical messages. It was also very popular during the Cold War.

The text card consists of a matrix of 12 by 17 cells. Each cell consists of a letter or number in red and a word or phrase in black. Switching between the red and the black text is done by using the 'SWITCH ON' and 'SWITCH OFF' cells. Multiple ON and OFF cells are available in order to hide the frequency of their use.

Slidex was also used by various parts of the Dutch Army during the Cold War. The image on the right shows a typical Slidex as it was used in The Netherlands. At the bottom of the card it is identified as card '1' of series 'A'.
  
The message table (matrix) of the Slidex

The text card is removable and can easily be swapped for another one with a different layout. In the example above, the keywords on the text card are in alphabetical order, but this was not always the case. Slidex was introduced the British Army around 1943 and was used heavily during operation Overlord in June 1944. It was easily broken by the Germans using nothing but the intercepted radio messages. Nevertheless, it provided sufficient security for tactical messages.

Slidex Flap and lock of the Slidex Opened Slidex The message table (matrix) of the Slidex British Crown logo Slidex detail User instructions inside a pocket User manuals
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Slidex
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Flap and lock of the Slidex
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Opened Slidex
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The message table (matrix) of the Slidex
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British Crown logo
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Slidex detail
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User instructions inside a pocket
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User manuals

East European variant
Although it is commonly assumed that Slidex was introduced by the British Army, the Russians started using it at about the same time. Later, during the Cold War, its use spread out to the other states of the Warsaw Pact, such as the former DDR (East-Germany). The common (East) German name for the system was Sprechtafeln or Gesprächstabellen (conversation tables).

 More information
  
Close-up of the matrix on page 1
References
  1. Louis Kruth, The Slidex RT Code
    Cryptologia, Volume 8, Issue 2, April 1984, p. 163-172
Further information
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