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)

Example:

Each character of the plain message must be present in the grid otherwise it can not be encoded.

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

Example: D in position 1,4 (line 1 column 4) corresponds to 1 then 4 shots and so DCODE translates to . .... . ... ... .... . .... . .....

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.

Example: To decode the message .... ..... . ... ....., count the dots (the taps): 4 4 1 1 3 5, rewrite in groups of 2 (4,4) (1,1) (3,5) and translate these coordinates into letters, respectively T,A,P, so TAP is the message in plain text.

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.