Transposition Cipher

Transposition cipher encryption consists of writing the plaint text in an array and then arranging the columns of this array according to a defined permutation. If there are empty boxes, fill them with a neutral letter (such as X).

The permutation key is a series of numbers (which can be generated from a word) that indicates the order in which the columns are arranged.

Example: The word KEY is used to generate the permutation 2,1,3:

Example: Encrypt DCODE with the key KEY (permutation 2,1,3).

The encrypted message consists of the table after swapping the columns in the order of the key.

Example: The encrypted message is CDOEDX

Transposition cipher decryption is identical to encryption except that the order of the columns is changed.

Example: The encryption used the permutation 2,1,3 to get the message CDOEDX:

The message consists of the letters of the original message but in a different order.

The index of coincidence is identical to that of the one of the language of the plaintext.

It is possible to test all the permutations if the key is not too long, but the most effective method is to have or try to guess a word from the plain text and to deduce the permutations of the columns.

The transposition cipher is, along with the substitution cipher, one of the most used bricks for more elaborate ciphers. There are dozens of ciphers that use it like ADFGVX, Amsco, Double Transposition, Redefence, etc.