Double Transposition Cipher

Double transposition encryption uses twice a transposition cipher, usually the first transposition is by columns, and the second by rows.

Example: Encrypt the message DCODE with twice the key KEY. The grid (1) is completed with X and permuted a first time (2)

With the message found after the first permutation, then perform a second transposition, but with the rows. The ciphertext is then obtained by reading the grid in rows from left to right and from top to bottom.

Example: The encrypted intermediate message is CDOEDX (3) and the final encrypted message is OECDDX (4):

Double transposition decryption requires knowing the two permutation keys and the type of transposition for each (row or column)

Example: The crypted message is OECDDX has been transposed with 1 column transposition then with 1 line transposition with two identical keys: KEY.

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

Example: The grid (1) becomes after inverse permutation in rows (2)

Example: The intermediate message CDOECX undergoes a second inverse permutation in columns (3) which gives the original starting grid (4) and the clear message 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/checkerboard, permute the columns in order to find and recreate words on each rows, beginning with common bigrams (such as TH, HE, IN, etc.).

The lines should not follow each other, do not try to read words on several lines.

The second permutation is deductible from the line pieces that were created with the first permutation.

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

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