Tools to decrypt/encrypt with double transposition cipher, as its names indicates, it uses two transposition ciphers, with one or two keys, and applied on lines or columns or both.

Double Transposition Cipher - dCode

Tag(s) : Cryptography,Transposition Cipher

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Tools to decrypt/encrypt with double transposition cipher, as its names indicates, it uses two transposition ciphers, with one or two keys, and applied on lines or columns or both.

Encryption uses twice a transposition cipher, usually the first transposition is in columns, and the second in lines.

One wants to crypt DCODE with twice the key KEY

\ | K | E | Y |
---|---|---|---|

1 | D | C | O |

2 | D | E | X |

The grid is filled with X and permutated a first time

\ | E | K | Y |
---|---|---|---|

1 | C | D | O |

2 | E | D | X |

The intermediate ciphered message is CDOEDX. One makes a second transposition, but with lines

\ | 1 | 2 |
---|---|---|

K | C | D |

E | O | E |

Y | D | X |

The grid becomes, after the second permutation

\ | 1 | 2 |
---|---|---|

E | O | E |

K | C | D |

Y | D | X |

The ciphered text is then obtained by reading the grid in lines, from left to right and from top to bottom.

The ciphered message is then OECDDX

Decryption requires to know both keys and the kind of transposition used for each (line or column)

Let the crypted message be OECDDX, transposed with 1 column transposition and 1 line transposition with two identical keys : KEY.

The crypted message gets two reversed transposition, in the opposite order of the original order, to get back the plain text.

Si the grid is

\ | 1 | 2 |
---|---|---|

E | O | E |

K | C | D |

Y | D | X |

It becomes, avec a first reversed permutation in lines

\ | 1 | 2 |
---|---|---|

K | C | D |

E | O | E |

Y | D | X |

The intermediate message is then CDOECX, it gets another reversed permutation, in column

\ | E | K | Y |
---|---|---|---|

1 | C | D | O |

2 | E | D | X |

The grid becomes

\ | K | E | Y |
---|---|---|---|

1 | D | C | O |

2 | D | E | X |

The original plain text is DCODEX.

Frequency analysis should be normal and the ciphered message has an index of coincidence similar to the language of the plain text. Its number of letters is not a prime number.

It is possible to find the key length by analyzing the prime decomposition of the text length.

By writing the text in the grid, one can permute the columns in order to find words on each lines, beginning with common bigrams (such as TH, HE, IN, etc.).

To find the second permutation, it is easier, because lines are already in plain text.

It is possible to shuffle lines or columns in the desired order.

No date of known author.

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transposition,double,key,two,permutation,combination,line,column,grid,anagram,disorder

Source : http://www.dcode.fr/double-transposition-cipher

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