Row Transposition Cipher

Row Transposition Cipher is a #Permutation/Transposition Cryptographic #algorithm that arrange the plaintext in an order specified by a key (a sequence of numbers or a word) accordingly in a row to produce a ciphertext. Initially, the plaintext should be written out in rows according to the key size (determine how many columns should the plaintext be written).

For example, given the example plaintext “attack postponed until two am” with a key “3 4 2 1 5 6 7”. Write the plaintext according to the key size like the following (fill in alphabets if there are spaces left):

Key:        3 4 2 1 5 6 7
Plaintext:  a t t a c k p
            o s t p o n e
            d u n t i l t
            w o a m x y z

The produced ciphertext is ATATCKP PTOSONE TNDUILT MAWOXYZ.

A Cryptanalysis could be done to this cipher by simply analysing the number of characters used in the ciphertext and guess the number of rows it’s using.