40.1 Kerberos Terminology

The following glossary defines some Kerberos terminology.

credential

Users or clients need to present some kind of credentials that authorize them to request services. Kerberos knows two kinds of credentials—tickets and authenticators.

ticket

A ticket is a per-server credential used by a client to authenticate at a server from which it is requesting a service. It contains the name of the server, the client's name, the client's Internet address, a time stamp, a lifetime, and a random session key. All this data is encrypted using the server's key.

authenticator

Combined with the ticket, an authenticator is used to prove that the client presenting a ticket is really the one it claims to be. An authenticator is built of the client's name, the workstation's IP address, and the current workstation's time all encrypted with the session key only known to the client and the server from which it is requesting a service. An authenticator can only be used once, unlike a ticket. A client can build an authenticator itself.

principal

A Kerberos principal is a unique entity (a user or service) to which it can assign a ticket. A principal consists of the following components:

• Primary—the first part of the principal, which can be the same as your username in the case of a user.

• Instance—some optional information characterizing the primary. This string is separated from the primary by a /.

• Realm—this specifies your Kerberos realm. Normally, your realm is your domain name in uppercase letters.

mutual authentication

Kerberos ensures that both client and server can be sure of each others identity. They share a session key, which they can use to communicate securely.

session key

Session keys are temporary private keys generated by Kerberos. They are known to the client and used to encrypt the communication between the client and the server for which it requested and received a ticket.

replay

Almost all messages sent in a network can be eavesdropped, stolen, and resent. In the Kerberos context, this would be most dangerous if an attacker manages to obtain your request for a service containing your ticket and authenticator. He could then try to resend it (replay) to impersonate you. However, Kerberos implements several mechanisms to deal with that problem.

server or service

Service is used to refer to a specific action to perform. The process behind this action is referred to as a server.