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Polybius Cipher

Tool to decrypt/crypt Polybius automatically. Polybius cipher (or Polybius Square) consists in replacing each letter by its coordinates of its position in a grid (usually a square). This encryption system can be used with a key which generates a deranged alphabet to fill the grid.

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Polybius Cipher -

Tag(s) : Cryptography, 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)

Tool to decrypt/crypt Polybius automatically. Polybius cipher (or Polybius Square) consists in replacing each letter by its coordinates of its position in a grid (usually a square). This encryption system can be used with a key which generates a deranged alphabet to fill the grid.

## Answers to Questions

### 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
.

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 is then 14,13,34,14,15

### How to decrypt Polybius cipher?

Decryption requires to know the grid and consists in a simple 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 line, 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.

Coordinates may have at most 25 distinct values.

### How to decipher Polybius without the grid?

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

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

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.

### When Polybius Cipher have been invented?

The greek historian Polybius described it in 150 before JC.

## Source code

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## Questions / Comments

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