Tap Code Cipher

The tap code cipher uses a grid of letters, usually 5x5, containing 25 of the 26 letters of the alphabet (a letter is omitted, often the J, the K or the Z).

The coordinates (row, column) of the cells of the grid are numbered from 1 to 5 and thus any cell has an quivalent pair of digits (row, column)

The principle of the tap code is to strike a number of hits corresponding to the coordinates of each character.

The decryption of the tap code requires knowing the grid and counting the numbers of tap/knock by arranging them in groups of 2 forming the coordinates (line, column) of each letter of the plain message.

The message is composed of a single character repeated between 1 and 5 times, a separator (like /) can be used, similar to the Morse.

The message can be in the form of a sound or a or light, again repetitive.

The name tap or knock is the onomatopoeia of the noise when the code is tapped or knocked on a surface such as a wall.

By default the grid is often the same: composed of the alphabet but without the letter K or the letter J (sometimes the letter Z), testing these few grids should be enough, otherwise to use a random grid and use the mono-alphabetic substitution decryption tool.

The grid may have a different size or use a shuffled alphabet.

A 6x6 grid containing 36 characters (26 letters and 10 digits) can be used to encode alphanumeric messages containing words and numbers.

Any communication containing 6, 7, 8 or more successive taps may indicate something special, a start of a message, an end of a message, a mistake made, etc.

The code is certainly very old, but there is no specific date. It has been used by prisoners in jails for centuries.

A little more recently, this code was used during the Vietnam War by a certain Captain Carlyle (Smitty) Harris.