Alice and Bob

Suppose Alice wants to send a secret message s to Bob. If Eve intercepts the message, then she can read it and it will not be secret anymore. Thus, Alice and Bob should agree on a two ways protocol that will turn the secret message s into a public message p. This is called encryption. Reversing the operations will allow Bob to recover s from p. For example, Alice would shift the letters of the message in alphabetical order and Bob will simply do the same thing in the reverse order (Caesar cipher). The Advanced Encryption Standard (AES) protocol does the same thing in a more complicated way, but this is not the subject of this course.

If Eve knows the two ways protocol, then she can derive s from p as easily as Bob does and the message will not stay secret anymore. The solution is to use a protocol with a parameter, the key. Then, Alice and Bob can make their protocol public as long as they keep secret their key k. For example, the protocol could be “replacing each letter in the message with the letter that is k places further down the alphabet”. Again, AES does the same thing in a more complicated way.