Tap code: Difference between revisions

Revision as of 23:36, 5 September 2007

Revision as of 23:00, 18 September 2007

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The '''Tap Code''' is a [[code (cryptography)|codecipher]], commonly used by prisoners in jail to communicate with one another. The method of communicating is usually by "tapping" either the metal bars or the walls inside the cell, hence its name. It is a very simple codecipher, not meant to avoid interception, since the messages are sent in [[cleartext]].

It was reportedly invented in June [[1965]] by four [[POW]]s imprisoned in [[Hoa Lo]], [[Vietnam]]: [[Captain]] Carlyle ("Smitty") Harris, [[Lieutenant]] Phillip Butler, Lieutenant Robert Peel and [[Lieutenant Commander]] Robert Shumaker. Harris remembered an [[United States Air Force|Air Force]] instructor who had shown him a codecipher based on a five-by-five alphabet [[matrix]] (a [[Polybius square]]), as shown on the graph below. Each letter was communicated by tapping two numbers: the first designated the row (horizontal) and the second designated the column (vertical). The letter "X" was used to break up sentences and the letter "C" replaced the letter "K".

The Tap Code is featured in [[Arthur Koestler]]'s classic work ''[[Darkness at Noon]]'', which was published in 1941. The prisoners refer to it as the knock code, but the technique is still the same.

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For example, to specify the letter "A", you would tap roughly the following: . .

Or to communicate the word "WATER" the codecipher would be the following (the time between each pair of numbers is smaller than the one between two different letters):

..... .. . . .... .... . ..... .... ..

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Because of the difficulty and length of time required for specifying a single letter, most prisoners devised [[abbreviations]] and [[acronyms]] for common items or phrases, such as "GN" for ''Good Night'', or "GBU" for ''God Bless You''.

There is however the possibility to do with less taps. First youone can arrange the letters in a diagonal fashion. The code shown below never needs more than 8 taps per letter opposed to letters with 9 and 10 taps in the "standard" tap code. Second you can exploit letter frequency as in [[Morse code]]. Obviously E should be the code with two taps, not A.