Kerberos: The Definitive Guide

Authentication

Simply put, authentication is the process of verifying the identity of a particular user. To authenticate a user, the user is asked for information that would prove his identity. This information can fall into one or more of three categories: what he knows, what he has, or what he is. These categories are referred to as factors .

The first factor, what he knows, is the most common factor used in authentication today. A secret password is generated when the user is granted access to a machine or network. That secret can either be generated by the user himself, by choosing his own password and giving it to the system administrator when he grants the user access, or automatically through some process that generates random passwords.

The second factor, what he has, is a less common but more secure alternative. An example of this type of authentication is the widely deployed RSA SecurID token. The SecurID token is a small electronic device that has an embedded encryption key and an LCD display. Every minute, an algorithm runs inside the device and updates the LCD display with a new six-digit combination. Only the person who possesses the device can tell what the correct password is. Other systems, such as smart card systems, operate on similar principles.

The third factor, what he is, enters into the realm of biometrics. Since all humans have distinguishing characteristics, biometrics measures the physical properties of some portion of our body and uses that information ...