Tap Code Cipher

Tap Code, also known as tap cipher, is an encryption method that uses sequences of taps to represent letters in a grid/matrix.

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

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 (row, 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 can have a different size, different content such as a mixed alphabet or even reverse the writing of the coordinates (row-column or column-row).

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

Any communication/tapping 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.

A source code for tap code encryption would look like // Pseudo-code
function tapCodeEncrypt(message) {
tapCodeGrid = [
["A", "B", "C", "D", "E"],
["F", "G", "H", "I", "J"],
["L", "M", "N", "O", "P"],
["Q", "R", "S", "T", "U"],
["V", "W", "X", "Y", "Z"]
]
encryptedMessage = "";
for each character in message {
for i from 0 to 4 {
for j from 0 to 4 {
if (tapCodeGrid[i][j] == character) {
encryptedMessage += (i+1)*"." + " " + (j+1)*"." + " "
}
}
}
}
return encryptedMessage
}