Polybius Cipher

Polybius square uses a 5x5 grid filled with letters for encryption.

Example: To crypt DCODE with the grid

A password can be used to generate a deranged alphabet that fills the grid.

As latin alphabet has 26 letters and the grid has 25 cells, a letter to remove is chosen, usually it's J, V, W or Z which are deleted. The order of the letters in the grid can be modified using a key to generate a deranged alphabet.

The encryption phase is a substitution of each letter by its coordinates (line, column) in the grid.

Example: D is located line 1, column 4, so coded 14; C is located line 1, column 3, it is coded 13. The ciphered message DCODE is then 14,13,35,14,15

Polybius decryption requires to know the grid and consists in a substitution of couples of coordinates by the corresponding letter in the grid.

Example: The message to decrypt is 351332542114 with the grid (created with DCODE as key and without letter J):

Split the message in bigrams, couples of numbers that are the coordinates of each plain text letter.

Example: 35,13,32,54,21,14, 35 stands for 3rd line, 5th column, so letter P, and so on. The plain message is POLYBE.

The ciphered message is constituted of couples of coordinates (generally these are digits from 1 to 5) and so has an even number of characters (the possible pairs are: 11, 12, 13, 14, 15, 21, 22, 23, 24, 25, 31, 32, 33, 34, 35, 41, 42, 43, 44, 45, 51, 52, 53, 54, 55).

Coordinates may have at most 25 distinct values.

References to Greece (Polybius comes from its author Πολύβιος / Polúbios in Greek) are a clue.

Polybius is a substitution by bigrams, replace each couple of coordinates by a random letter (there should be at most 25 distinct ones) and try a monoalphabetical desubstitution.

It is possible to use a grid of another size, may be rectangular. It is also possible to use other coordinates notation, for example column or line name other than digits from 1 to 5, but also to note then in column-line rather than line-column.

The author (Polybius) had proposed to transmit coded messages remotely, for example, using torches. N in the right hand and M in the left hand for the coordinates N, M for example.

The Nihilists cipher is a variant using an over-encryption of the Polybe code.

The greek historian Polybius described it in 150 before JC.