The thing about authentication as described in Section 1.1, “Confirming User Identities”
is that every secure application requires at least a password to access it. Without a central identity store and every application maintaining its own set of users and credentials, a user has to enter a password for every single service or application he opens. This can require entering a password several times a day, maybe even every few minutes.
Maintaining multiple passwords, and constantly being prompted to enter them, is a hassle for users and administrators. Single sign-on is a configuration which allows administrators to create a single password store so that users can log in once, using a single password, and be authenticated to all network resources.
Red Hat Enterprise Linux supports single sign-on for several resources, including logging into workstations, unlocking screen savers, and accessing secured web pages using Mozilla Firefox. With other available system services such as PAM, NSS, and Kerberos, other system applications can be configured to use those identity sources.
Single sign-on is both a convenience to users and another layer of security for the server and the network. Single sign-on hinges on secure and effective authentication. Red Hat Enterprise Linux provides two authentication mechanisms which can be used to enable single sign-on:
Kerberos-based authentication, through both Kerberos realms and Active Directory domains
Smart card-based authentication
Both of these methods create a centralized identity store (either through a Kerberos realm or a certificate authority in a public key infrastructure), and the local system services then use those identity domains rather than maintaining multiple local stores.