What is difference between monoalphabetic and polyalphabetic cipher?

Answer 1

In a monoalphabetic cipher, our substitution characters are a

random permutation of the 26 letters of the alphabet:

plaintext letters: a b c d e f .....

substitution letters: t h i j a b .....

• The key now is the sequence of substitution letters. In other

words, the key in this case is the actual random permutation of

the alphabet used.

For a particular alphabet, only one substitution can be used

example: if the substitution letter for 'a' is 't' and for 'd' it is 'p'

then if the keyword happens to be "tpp"..... then we can encrypt the message as "add"(note that the substitution letter for 'd' is only 'p'..

In a monoalphabetic cipher, the same substitution rule is used for

every substitution. In a polyalphabetic cipher, the substitution

rule changes continuously from letter to letter according to the

elements of the encryption key.

Example for polyalphabetic:

key: abracadabraabracadabraabracadabraab

plaintext: canyoumeetmeatmidnightihavethegoods

ciphertext: CBEYQUPEFKMEBK.....................

• The Vigenere cipher is an example of a polyalphabetic cipher.