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Mono-alphabetic Substitution
Caesar Cipher
2000 years ago Julius Caesar used a simple substitution cipher, now known as the Caesar cipher
first attested use in military affairs (Gallic Wars)
replace each letter by 3rd letter on, eg.
L FDPH L VDZ L FRQTXHUHG is
I CAME I SAW I CONQUERED
can describe this mapping (or translation alphabet) as:
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: EFGHIJKLMNOPQRSTUVWXYZABC
General Mono-alphabetic Substitution
rather than just shifting the alphabet could shuffle (jumble) the letters arbitrarily
each plain text letter maps to a different random cipher text letter
hence key is 26 letters long
have a total of 26! ~ 4 x 1026 keys!
but … can be cracked through frequency analysis
Poly-alphabetic Substitution Cipher
Vigenère Cipher
Blaise de Vigenère is generally credited as the inventor of the “polyalphabetic substitution cipher”
To improve security use many monoalphabetic substitution alphabets
Hence each letter can be replaced by many others
Use a key to select which alphabet is used for each letter of the message
ith letter of key specifies ith alphabet to use
Use each alphabet in turn
Repeat from start after end of key is reached