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Polybius Cipher

Tool to decode/encode with the Polybius square cipher automatically (with or without a grid and therefore with or without the keyword).

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Polybius Cipher -

Tag(s) : Substitution Cipher

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# Polybius Cipher

## Polybius Square Decoder

 Coordinates Order Line, Column (usual) Column, Line (unusual)

## Polybius Square Encoder

 Coordinates Order Line, Column (usual) Column, Line (unusual)

### What is the Polybius cipher? (Définition)

The Polybius cipher, also called Polybius square, is a substitution cipher using a square grid. Each character of the plain message is replaced by a couple of coordinates defining its position in the grid.

### How to encrypt using Polybius cipher?

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

Example: To crypt DCODE with the grid

1 2 3 4 5 \ A B C D E F G H I J K L M N O P Q R S T U V W X Y

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

As the 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 (row, column) in the grid.

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

### How to decrypt Polybius cipher?

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):

1 2 3 4 5 \ D C O E A B F G H I K L M N P Q R S T U V W X Y Z

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 row, 5th column, so letter P, and so on. The plain message is POLYBE.

### How to recognize Polybius ciphertext?

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.

### How to decipher Polybius without the grid?

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 substitution.

### What are the variants of the Polybius cipher?

It is possible to use a grid of another size, not necessarily square, maybe rectangular. It is also possible to use other coordinate notation, for example column or row names other than digits from 1 to 5, but also to note them in column-row rather than row-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.

### When was Polybius Cipher invented?

The greek historian Polybius described this method in 150 before JC.

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