In this cipher, the alphabet is the usual alphabet of Roman letters. Punctuation, spaces and capitalizations are ignored. It is reported that Caesar used an encryption based on the substitution A d, ... , Z c, given by the table below.
A | B | C | D | E | F | G | H | I | J | K | L | M |
d | e | f | g | h | i | j | k | l | m | n | o | p |
N | O | P | Q | R | S | T | U | V | W | X | Y | Z | |
q | r | s | t | u | v | w | x | y | z | a | b | c |
If we identify each letter with its corresponding number via the labeling is , is , ..., is , then Caesar's cipher is a cipher for which any letter is related to its enciphered letter by
where in the example above . More generally, a substitution cipher is a cipher for which the alphabet has been rearranged according to some fixed permutation. The Caesar cipher is a special case of this where the permutation is as indicated above.