These systems are used to authenticate a user to a server or service. Each system defines the following common set of principals. To ease the comparison of these systems, the following names will be used to define the common principals:
Username (U) - The identity of a user.
Hostname (H) - The identity of a host.
Client (C) - A host, acting on behalf of a user, requesting services of a host.
Server (S) - A host who provides services.
Certification Authorities (CA) - A server that is used to provide the authentication information. (This may not reside on a dedicated server.)