Other early attempts at cryptography (the science of data protection via encryption) used substitution. A substitution algorithm simply replaces each character in a message with another character. Caesar's cipher is an example of a substitution algorithm. To create these messages, you list the alphabet across a page and agree with the recipient on the starting letter-suppose you agree to tart with the fourth letter of the alphabet, D.Starting with this letter, you write down a new alphabet under the old,so it looks like this :