Kerberos: The Definitive Guide

Kerberos 4

The Kerberos 4 protocol is largely based on the Needham-Schroeder protocol, with two major changes.

The hosts involved in the Kerberos 4 protocol exchanges map directly to the principals involved in the Needham-Schroeder protocol. The authentication client is a Kerberos 4 user workstation, and the authentication server maps to a Kerberos 4 Key Distribution Center.

The first change to the Needham-Schroeder protocol reduced the amount of network messages sent between the client and the authentication server. The original Needham-Schroeder protocol did not have a dependence on a network time source, but the cost was an extra two message exchanges. The last two message exchanges in the Needham-Schroeder protocol establish that there is no man in the middle posing as the authentication server, and that the session key is not a replay. In the Kerberos 4 protocol, replay is thwarted through an authenticator message that is constructed of the local time of the client encrypted with the newly-negotiated session key of the connection. While this requires time synchronization between all hosts involved, it does reduce the number of network messages required per authentication exchange.

The second, more significant, change to the basic protocol creates the concept of a Ticket Granting Ticket, which allows users to authenticate to multiple application servers while entering their authentication secret only once. If the original Needham-Schroeder protocol were implemented as-is, a user ...