Question

What is difference between monoalphabetic and polyalphabetic cipher?

Answer 1

A monoalphabetic cipher mixes up the letters of the alphabet and keeps that same arrangement for the entire message. This is the type of cipher normally seen in newspaper cryptograms and is very easy to break. A polyalphabetic cipher, by contrast, uses more than one monoalphabetic cipher within the same message.

For example, if you use a Vigenere disk and leave the two disks stationary while encoding a message, that is a monoalphabetic cipher. If you rotate the disks while encoding the message, using a keyword, then that is a polyalphabetic cipher.