Computer Security :: Lessons :: Transposition
A transposition cipher performs some sort of permutation on the plaintext letters of the text to be encrypted. The rail fence technique is a simple example of transposition.
m e m a t r h t g p r y e t e f e t e o a a t
Cryptanalysis on the above message is simple as the ciphertext simply becomes "MEMATRHTGPRYETEFETEOAAT" and the ciphertext contains the same letter frequency as the plaintext. A slightly more complex transpostion scheme uses a rectangle.
Key: 4 3 1 2 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
Starting in column 1 and proceding down the column, the ciphertext becomes "TTNAAPTMTSUOAODWCOIXKNLYPETZ," which is slightly more complex than the rail fence technique, but still easy to break because of the duplicated letter frequency. To make this message more secure, you can use double transposition to encrypt the message a second time. Using the ciphertext we just generated and the same process, you will generate a new ciphertext of "NSCYAUOPTTWLTMDNAOIEPAXTTOKX" that is harder to cryptanalyze than single transposition.
Key: 4 3 1 2 5 6 7 Plaintext: t t n a a p t m t s u o a o d w c o i x k n l y p e t z
A rotor machine uses multiple transpositions to encrypt information. Each input of the plaintext message moves the rotors so all letters are mapped to a new ciphertext letter. The video below provides a good explanation of how rotor machines function.