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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.

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