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

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

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

Example: The grid becomes, after the second permutation

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

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

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

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

Example: The grid becomes

Example: The original plain text is DCODEX.

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.

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.

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

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