Kerberos: The Definitive Guide

Kerberos 5

If you look strictly at the feature set, Kerberos 5 is an evolution of Kerberos 4. The Kerberos 5 protocol contains all of the functionality present in the Kerberos 4 protocol, but with many extensions. However, from an implementation perspective, Kerberos 5 is a completely new protocol, and looks nothing like Kerberos 4 on the inside. In this section, we’ll examine the new features present in Kerberos 5 as well as the new infrastructure provided by the protocol to make these features work.

The Kerberos 4 protocol had its share of shortcomings: it had a rather obtuse structure (for example, instead of standardizing on one byte order, it had a flag to specify which byte order was used to send a particular message) and it wasn’t expandable, since many of its fields had fixed sizes. This limitation led to other problems, most notably the dependence on single-DES encryption keys. At the time that Kerberos 4 was developed, a brute-force attack against DES was still prohibitively expensive in terms of both resources and time. As computer speed continues to grow exponentially, it is now within the realm of well-funded adversaries to mount a brute-force attack against DES. Therefore, a more secure encryption algorithm with a longer encryption key size is needed. Unfortunately, since all of the fields in Kerberos 4 are fixed size, there is no way to retrofit Kerberos 4 with another encryption algorithm.

Another feature that users and administrators alike demanded from a new version ...