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Double Transposition Cipher

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.

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Double Transposition Cipher -

Tag(s) : Cryptography, Transposition Cipher

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# Double Transposition Cipher

## Double Transposition Encoder

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.

### How to encrypt using Double Transposition cipher?

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

Example: Encrypt DCODE with twice the key KEY

 D C O D E X

Example: The grid is filled with X and permutated a first time

 C D O E D X

Example: The intermediate ciphered message is CDOEDX. Make a second transposition, but with lines

1 2 \ C D O E D X

Example: The grid becomes, after the second permutation

1 2 \ O E C D D X

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

Example: The ciphered message is then OECDDX

### How to decrypt Double Transposition cipher?

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

Example: The crypted message is 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.

Example: Si the grid is

1 2 \ O E C D D X

Example: It becomes, avec a first reversed permutation in lines

1 2 \ C D O E D X

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

 C D O E D X

Example: The grid becomes

 D C O D E X

Example: The original plain text is DCODEX.

### How to recognize Double Transposition ciphertext?

A message encrypted by Double Transposition has a frequency analysis and a coincidence index normal.

The number of letters in the message is not a prime number.

### How to decipher Double Transposition without keys?

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

By writing the text in the grid, 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.

### What are the variants of the Double Transposition cipher?

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

### When Double Transposition have been invented?

No date of known author.