Columnar Transposition Cipher -- one of the classical pen & paper ciphers. The ciphertext is read from the columns, scrambling the order of the letters.

It is not secure when used as a standalone cipher, but it can be a powerful enhancement to other systems (eg. substitution ciphers). It expands the potential keyspace of the other cipher and thwarts many common substitution cipher

Column transposition encryption, also called column permutations, is a cryptographic technique that modifies the order of the letters of a previously written text in a table.

Encryption :

Write the plaintext in an array of N columns with N the size of the permutation key.

The text CRYPTOLOGY is encrypted with the permutation 1,3,4,2, of the keyword CODE, written in the tabble

Columns
1| 2|3 |4
C|O|D|E
___________
C|R|Y|P
T|O|L|O
G|Y|X|X

You may add a neutral letter (like X) to the empty box/es.

To get the permutation of the keyword code, you just have to write the letters of the keyword in alphabetical order. Once done, you may now rearrange the columns using the permutation.

Permuted columns

1|3|4 |2
C|D|E|O
___________
C|Y|P |R
T|L|O|O
G|X|X|Y

The encrypted message is then read in columns.

Ciphered text: CTGYLXPOXROY