Columnar Transposition Cipher is a fairly simple, easy to implement cipher. It is a transposition cipher that follows a simple rule for mixing up the characters in the plaintext to form the ciphertext.

How it works?

The key for the Columnar Transposition Cipher is a keyword e.g. GERMAN. The row length that is used is the same as the length of the keyword. To encrypt a piece of text, e.g.

defend the east wall of the castle

Then write it out in a special way in a number of rows (the keyword here is GERMAN):

G E R M A N
d e f e n d
t h e e a s
t w a l l o
f t h e c a
s t l e x x

In the above example, the plaintext has been padded so that it neatly fits in a rectangle. This is known as a regular columnar transposition. An irregular columnar transposition leaves these characters blank, though this makes decryption slightly more difficult. The columns are now reordered such that the letters in the key word are ordered alphabetically.

A E G M N R
n e d e d f
a h t e s e
l w t l o a
c t f e a h
x t s e x l

The ciphertext is read off along the columns:

nalcxehwttdttfseeleedsoaxfeahl