Password Based Login
As an introduction to security and cryptography, lets look at something it does extensively, namely to authenticate ourselves using passwords. It considers a computer on which many different people have accounts. Each of these people has an associated user name. For example, it might have a user named Elaine Benes, whose user name is ebenes, and a user named Cosmo Kramer, whose user name is cosmo. A user name is a quantity that uniquely identifies a person in this context (if there were two people named Cosmo Kramer, they would get different user names) and is not considered a secret.