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Kerberos: The Definitive Guide by Jason Garman

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Chapter 1. Introduction

Who are you? It’s a question with an obvious response, at least for people. Humans have the ability to distinguish one another through several senses; most commonly, we use our sense of vision to recognize people we have met before. We also can tell one another apart through other means, such as body language, speech patterns and accents, and shared secrets between people. It has even been shown that newborn babies can discern between their mother and other females solely through their scent. Our ability to recognize patterns in our surroundings provides us with this ability to determine the identity of, or authenticate , people we know.

However, when you bring a computer into the picture, the situation changes dramatically. Computers (at least today’s computers) don’t have eyes, ears, or noses. Even if they did, the current state-of-the-art in pattern recognition is still woefully inaccurate for widespread use. While there is a lot of research in this area, the most common method by far for authenticating people to computers is through passwords. A password, also known as a shared secret , is the one critical piece of information that determines whether the person behind the keyboard really is whom they claim to be. While humans sometimes use this shared secret method—for example, a secret handshake, or perhaps the knowledge of obscure trivia—computers almost exclusively use shared secrets to authenticate people.

There are two issues with passwords as used today for authentication. The first is a human problem. We don’t like to remember a long, complex string of numbers, letters, and maybe even symbols that make up a secure password. If left to our own devices, we use simple dictionary words or maybe even our spouses’ name or birthdate as passwords. Unfortunately, a “shared secret” that really isn’t a secret (such as your spouse’s name) is easily guessable by an attacker who wishes to impersonate you to the computer. This problem is exacerbated by the fact that, even within a company network, there are literally dozens of machines a person has access to, each of which requires its own password. As a general rule, as the number of passwords goes up, the quality of each password decreases.

The second issue is a technical problem. While the computer gives you the illusion of security by printing stars, or nothing at all, on the screen while you type your password, somehow that information must travel some communications network to a computer on the other end. The most common method that computers use to send passwords over the network is by sending the password in "clear text,” that is, unmodified. While this wouldn’t be a problem if each computer had a completely separate, dedicated connection to every other computer it wishes to communicate with, in reality, computer networks are a shared resource. Sending passwords over the network in the clear is analogous to standing in a crowded room shouting across the room to a friend standing on the other side.

Kerberos is a network authentication system that can help solve those two issues. It reduces the number of passwords each user has to memorize to use an entire network to one—the Kerberos password. In addition, Kerberos incorporates encryption and message integrity to solve the second issue, ensuring that sensitive authentication data is never sent over the network in the clear. By providing a secure authentication mechanism, Kerberos is an essential part of a total network security plan, providing clear benefits for both end users and administrators.

But before we go further, let’s take a step back and explore the history of Kerberos.


The word Kerberos originates from Greek mythology, which contains the legend of Cerberus. Cerberus guarded the realm of the underworld, ruled by Hades and his wife, Persephone. What Cerberus looked like depends on whom you ask; Hesiod claims that Cerberus has fifty heads, while Apollodorus describes him as a strange mixture of creatures with three dog-shaped heads, a serpent as a tail, and heads of snakes over his back. Cerberus is most often pictured as a creature with three heads. Either way, Cerberus was a vicious creature that few dared to challenge.

The Greeks believed that when a person dies, his soul is sent to Hades to spend eternity. While all souls were sent to Hades, those people who had led a good life would be spared the eternal punishment that those who had not would have to endure. Cerberus, as the gatekeeper to Hades, ensured that only the souls of the dead entered Hades, and he ensured that souls could not escape once inside.

As the gatekeeper to Hades, Cerberus authenticated those who attempted to enter (to determine whether they were dead or alive) and used that authentication to determine whether to allow access or not. Just like the ancient Cerberus, the modern Kerberos authenticates those users who attempt to access network resources.

Like every other great figure in mythology, Cerberus had a fatal flaw that enabled some clever people to pass through Cerberus to Hades. We’ll revisit the legend and discuss one such story and its modern counterparts in Chapter 6.

Finally, if the ancient mythological character was named Cerberus, why is the modern authentication system called Kerberos? Simply put, they are just different spellings of the same word. In order to provide a distinction between the ancient mythology and the present-day software system, we will refer to the mythological character as Cerberus and the modern software system as Kerberos.

Modern History

The modern-day origins of the Kerberos network authentication system are a bit more mundane than the ancient mythology of Cerberus. Kerberos began as a research project at the Massachusetts Institute for Technology (MIT) in the early 1980s. The MIT faculty at the time recognized that the explosion of widely available, inexpensive computers would transform the computing industry.

The time-sharing model

Traditionally, computers were a large, expensive, and centralized resource that end users accessed through dumb terminals connected via serial lines. This is called the time-sharing model (Figure 1-1).

Time-sharing model

Figure 1-1. Time-sharing model

The time-sharing model had some distinct administrative advantages. Since there was only one (albeit large) computer, maintaining user accounts and privileges was easy. To add a user account, the administrator simply created an entry in the machine’s password file and the user immediately had access. Deleting accounts or changing passwords was similarly easy; there was a single point of administration by definition, since there was only one machine involved.

In addition, the time-sharing model had no need for encryption of passwords during transit. Users connected to the machine via dedicated serial lines, and each user had their own dumb terminal connected to their own serial line. Since the communication medium was not shared, one user could not listen into the communications of another user without physically tapping their serial wire.

The client-server model

The advent of the packet-switched computer network dramatically changed how users interfaced with computers. With the computer network, users all had an inexpensive computer on their desk, connected to all of the other computers in the organization through the network. Since the desktop computers still were not very powerful, specialized, more powerful server computers were used to provide services such as file storage, printing, and electronic mail to users.

This new computing model, widely referred to as client-server , provided some distinct advantages to both users and administrators (Figure 1-2). End users enjoyed increased usability since the desktop computers, while not very powerful, were powerful enough to present menu-driven interfaces to the resources of the server machines; a marked improvement over typing cryptic commands on a dumb terminal or line printer.

Client-server model

Figure 1-2. Client-server model

However, the faculty at MIT recognized that this new model required a dramatic new software architecture and a new way of thinking about computing: a way of thinking that recognized that computing power, rather than being centralized, was now distributed throughout the entire campus.

A major problem that the advent of personal computers and the network presented was that the end-user clients could no longer be trusted. In the traditional time-sharing model, end users only had a dumb terminal, which had no processing power at all. Since the computer all of the users shared was under the total control of the system administrators, it could be trusted.

Now, with inexpensive personal computers on all of the end-users desks, the system administrators no longer had total control over all of the computing power; indeed, end users could modify any part of the software on their personal machine they wanted, going as far as replacing the operating system itself. Since the end users now controlled their own computer, they could spoof messages to other machines, listen in on connections between other machines on the network, and impersonate the identities of other users.

Project Athena

Enter Project Athena. Project Athena was established with support from a consortium of computer vendors in May 1983 with a five-year timeline. Athena’s focus was to develop strategies and software for integrating computers into MIT’s curriculum. In particular, Athena was designed from the start as a networked, client-server system. While Project Athena had a decidedly educational focus, the end result included several software packages that are still in wide use today.

As mentioned earlier, passing passwords over a network in clear text is akin to shouting in a crowded room. However, that is exactly how software of the day functioned: the standard remote login protocols such as Telnet and rlogin sent user credentials over the network in clear text. Even worse, other server software, such as rlogin, blindly believed the identity that the client computer sent to it. Thus, unscrupulous users could (trivially) write fake rlogin clients that would be able to impersonate anyone else on the network. This was unacceptable in an academic environment, where dishonest users could listen to network messages for other users’ passwords or steal others’ work.

In addition, the now decentralized nature of the computing resources presented the problem that every user had access to several computers on the network, each requiring its own username and password. This model contrasted with the older timesharing model, in which end-user machines were connected to one centralized computer via a dedicated serial line, and only one login was required.

To solve these problems, Project Athena developed the Kerberos protocol for network authentication. The Project Athena Technical Plan outlined the major goals of the new service; namely, to extend the services of authentication typically found in large time-sharing systems to a distributed network consisting of servers and untrusted end-user client machines. The new authentication system had to centralize the trust into machines that were tightly controlled and monitored, and encrypt any sensitive transactions between these authentication servers and other machines on the network.

Many other packages also came out of the efforts of Project Athena. Project Athena developed the X Window System, which is now used as the basis for every Unix system’s Graphical User Interface. Other packages that relate to distributed computing include the Hesiod distributed name service, and the Moira distributed network administration system.

The combination of Project Athena and similar projects at other large universities across the U.S. and the world provided these institutions with advanced single-sign-on network infrastructure across all major network protocols, including electronic mail, file sharing, and even instant messaging. By using Kerberos and applications that support Kerberos authentication in your own network, you can achieve this goal as well.

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