Columnar Transposition Cipher

Column transposition encryption writes plaintext in a rectangular array of N columns (inline fill) with N the size of the permutation key.

Example: The text COLUMNS is encrypted with the permutation 1,3,2 of the key word COL, it is written in the table

Fill in the empty boxes with a neutral letter (like X).

The encrypted message is then read in columns

Example: CUSLNXOMX

It is possible to read line by line, in which case the encrypted message would be CLOUNMSXX

Decryption by Columnar Transposition is similar to encryption. The difference lies in the writing in the table (in rown or in column according to the reading method used during the encryption), as well as in the order of the columns which are permuted before being sorted again in ascending order.

Example: A permutation 1,3,2 was used to obtain the message CUSLNXOMX (reading by columns):

The message consists of the transposed / swapped letters, so it has all the letters of the original message but in a different order.

The coincidence index after transposition of columns is unchanged from that of the plain text language.

For short permutations (up to 5-6 letters), a brute-force algorithm can test all permutations.

Otherwise, by knowing a word of the plain text (if possible with unusual letters), it is possible to find the position of its letters and to deduce the compatible columns permutations.