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.
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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.
Subtitution: each letter retains its position but changes its identity, Transposition: each letter retains its identity but changes its position.
Caesar Cipher
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The substitution method undoes the chain rule.