Columnar Transposition Cipher is a #Permutation/Transposition Cryptography #algorithm that arrange plaintext in an order specified by a key (a sequence of numbers or a word) accordingly in a column to produce ciphertext. The key size determines how long should the plaintext be written in a row before proceed to a new line.
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 APTMTTNAAODWTSUOCOIXKNLYPETZ.
A Cryptanalysis could be done to this cipher by simply analysing the number of characters used in the ciphertext and guess the number of columns it’s using.