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From the Book: I have written this book partly to correct a mistake. Seven years ago I wrote another book: Applied Cryptography. In it, I described a mathematical utopia: algorithms that would keep your deepest secrets safe for millennia, protocols that could perform the most fantastical electronic interactions-unregulated gambling, undetectable authentication, anonymous cash-safely and securely. In my vision cryptography was the great technological equalizer; anyone with a cheap (and getting cheaper every year) computer could have the same security as the largest government. In the second edition of the same book, written two years later, I went so far as to write: "It is insufficient to protect ourselves with laws; we need to protect ourselves with mathematics." It's just not true. Cryptography can't do any of that. It's not that cryptography has gotten weaker since 1994, or that the things I described in that book are no longer true; it's that cryptography doesn't exist in a vacuum. Cryptography is a branch of mathematics. And like all mathematics, it involves numbers, equations, and logic. Security, palpable security that you or I might find useful in our lives, involves people: things people know, relationships between people, people and how they relate to machines. Digital security involves computers: complex, unstable, buggy computers. Mathematics is perfect; reality is subjective. Mathematics is defined; computers are ornery. Mathematics is logical; people are erratic, capricious, and barely comprehensible. The error of Applied Cryptography is that I didn't talk at all about the context. I talked about cryptography as if it were The Answer. I was pretty naive. Theresult wasn't pretty. Readers believed that cryptography was a kind of magic security dust that they could sprinkle over their software and make it secure. That they could invoke magic spells like "128-bit key" and "public-key infrastructure." A colleague once told me that the world was full of bad security systems designed by people who read Applied Cryptography. Since writing the book, I have made a living as a cryptography consultant: designing and analyzing security systems. To my initial surprise, I found that the weak points had nothing to do with the mathematics. They were in the hardware, the software, the networks, and the people. Beautiful pieces of mathematics were made irrelevant through bad programming, a lousy operating system, or someone's bad password choice. I learned to look beyond the cryptography, at the entire system, to find weaknesses. I started repeating a couple of sentiments you'll find throughout this book: "Security is a chain; it's only as secure as the weakest link." "Security is a process, not a product." Any real-world system is a complicated series of interconnections. Security must permeate the system: its components and connections. And in this book I argue that modern systems have so many components and connections-some of them not even known by the systems' designers, implementers, or users-that insecurities always remain. No system is perfect; no technology is The Answer. This is obvious to anyone involved in real-world security. In the real world, security involves processes. It involves preventative technologies, but also detection and reaction processes, and an entire forensics system to hunt down and prosecute the guilty. Security is not a product; it itself is a process. And if we're ever going to make our digital systems secure, we're going to have to start building processes. A few years ago I heard a quotation, and I am going to modify it here: If you think technology can solve your security problems, then you don't understand the problems and you don't understand the technology. This book is about those security problems, the limitations of technology, and the solutions. Read this book in order, from beginning to end. No, really. Many technical books are meant to skim, bounce around in, and use as a reference. This book isn't. This book has a plot; it tells a story. And like any good story, it makes less sense telling it out of order. The chapters build on each other, and you won't buy the ending if you haven't come along on the journey. Actually, I want you to read the book through once, and then read it through a second time. This book argues that in order to understand the security of a system, you need to look at the entire system-and not at any particular technologies. Security itself is an interconnected system, and it helps to have cursory knowledge of everything before learning more about anything. But two readings is probably too much to ask; forget I mentioned it. This book has three parts. Part 1 is "The Landscape," and gives context to the rest of the book: who the attackers are, what they want, and what we need to deal with the threats. Part 2 is "Technologies," basically a bunch of chapters describing different security technologies and their limitations. Part 3 is "Strategies": Given the requirements of the landscape and the limitations of the technologies, what do we do now I think digital security is about the coolest thing you can work on today, and this book reflects that feeling. It's serious, but fun, too. Enjoy the read.