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.
