Monoalphabetic Substitution Cipher in which every alphabet have unique alphabet and we can not use same for another alphabet in cipher text.

Polyalphabetic cipher was invented by Johannes Trithemius in the 15th Century.

To remove this limitation, Polyalphabet cipher is introduced in which we can use alphabet for any  number.

For example,

                                                                       fig 1

Plain text: nikhil

cipher text: NJMKMQ

In this cipher technique, Trithemius’ idea was to start at the column headed by “A”, find the plaintext letter down the far left column, and encrypt this to the ciphertext letter in the first column. You would then move to the next column, and so on.

In the above fig 1 , M is used as k as well as i.