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.

Polybius Cipher - dCode

Tag(s) : Cryptography, Substitution Cipher

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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.

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

One wants to crypt DCODE with the grid

\ | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|

1 | A | B | C | D | E |

2 | F | G | H | I | J |

3 | K | L | M | N | O |

4 | P | Q | R | S | T |

5 | U | V | W | X | Y |

As latin alphabet has 26 letters, one has to choose a letter to remove, usually it's J, V, W or Z which are deleted (here the Z).

The order of the letters in the grid can be modified, often one uses a key to generate a deranged alphabet.

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

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

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

Let the message be 351332542114 to decrypt with the grid (created with DCODE as key and without letter J):

\ | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|

1 | D | C | O | E | A |

2 | B | F | G | H | I |

3 | K | L | M | N | P |

4 | Q | R | S | T | U |

5 | V | W | X | Y | Z |

One splits the message in bigrams, couples of numbers are the coordinates of each plain text letter.

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.

Coordinates may have at most 25 distinct values.

Polybius is in fact a simple substitution, one can 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.

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polybius,square,nihilist,russia,11,12,13,14,15,21,22,23,24,24,25,31,32,33,34,35,41,42,43,44,45,51,52,53,54,55

Source : http://www.dcode.fr/polybius-cipher

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