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Slidex
Maxtrix-based coding of fixed messages - wanted item

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

 
Russian 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).
 
The image on the right shows a Slidex variant with 100 cells, that was used by the Nationale Volksarmee (NVA, Peoples Army) in the DDR. It was issued in 1969 and is called Parolen- und Gesprächstabelle (expressions and conversation table). Apparently they used a series of different tables, as this one is called SERIE 'E' and 'F'.

Inside the book are two pages. The first page contains a 10 x 10 matrix of small rectangles (cells). Inside each cell is an expression or a complete sentence. At the bottom left of the page it is marked as 'SERIE E'.
  
Close-up of the matrix on page 1

Along the left side and along the top of the page is an orange plastic sheet, that allows it to be written with a pen or pencil. In most cases, letters or letter pairs would be written before each row and above each column. The combination of a row/column pair would then identify the appropriate cell. In order to further obscure the contents of the message, a numerical table on the second page was used to convert the letters (used for the rows and columns) into numbers. This page is marked 'SERIE F'.
 
The red code book as used by the East German Army (NVA) during the Cold War. Close-up of the cover of the East German Parolen- und Gesprächstabellen. The matrix on page 1 Close-up of the matrix on page 1 The translation table on page 2 Close-up of the translation table on page 2 Close-up of the bottom right corner of page 2 Typical view of the book

 
References
  1. Louis Kruth, The Slidex RT Code
    Cryptologia, Volume 8, Issue 2, April 1984, p. 163-172

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