Practical Cryptography

Niels Ferguson and Bruce Schneier

Table of Contents

Preface

In the past decade, cryptography has done more to damage the security of digital systems than it has to enhance it. Cryptography burst onto the world stage in the early 1990s as the securer of the Internet. Some saw cryptography as a great technological equalizer, a mathematical tool that would put the lowliest privacy-seeking individual on the same footing as the greatest national intelligence agencies. Some saw it as the weapon that would bring about the downfall of nations when governments lost the ability to police people in cyberspace. Others saw it as the perfect and terrifying tool of drug dealers, terrorists, and child pornographers, who would be able to communicate in perfect secrecy. Even those with more realistic attitudes imagined cryptography as a technology that would enable global commerce in this new on-line world.

Ten years later, none of this has come to pass. Despite the prevalence of cryptography, the Internet's national borders are more apparent than ever. The ability to detect and eavesdrop on criminal communications has more to do with politics and human resources than mathematics. Individuals still don't stand a chance against powerful and well-funded government agencies. And the rise of global commerce had nothing to do with the prevalence of cryptography.

For the most part, cryptography has done little more than give Internet users a false sense of security by promising security but not delivering it. And that's not good for anyone except the attackers.

The reasons for this have less to do with cryptography as a mathematical science, and much more to do with cryptography as an engineering discipline. We have developed, implemented, and fielded cryptographic systems over the past decade. What we've been less effective at is converting the mathematical promise of cryptographic security into a reality of security. As it turns out, this is the hard part.

Too many engineers consider cryptography to be a sort of magic security dust that they can sprinkle over their hardware or software, and which will imbue those products with the mythical property of "security." Too many consumers read product claims like "encrypted" and believe in that same magic security dust. Reviewers are no better, comparing things like key lengths and on that basis, pronouncing one product to be more secure than another.

Security is only as strong as the weakest link, and the mathematics of cryptography is almost never the weakest link. The fundamentals of cryptography are important, but far more important is how those fundamentals are implemented and used. Arguing about whether a key should be 112 bits or 128 bits long is rather like pounding a huge stake into the ground and hoping the attacker runs right into it. You can argue whether the stake should be a mile or a mile-and-a-half high, but the attacker is simply going to walk around the stake. Security is a broad stockade: it's the things around the cryptography that make the cryptography effective.

The cryptographic books of the last decade have contributed to that aura of magic. Book after book extolled the virtues of, say, 112-bit triple-DES without saying much about how its keys should be generated or used. Book after book presented complicated protocols for this or that without any mention of the business and social constraints within which those protocols would have to work. Book after book explained cryptography as a pure mathematical ideal, unsullied by real-world constraints and realities. But it's exactly those real-world constraints and realities that mean the difference between the promise of cryptographic magic and the reality of digital security.

Practical Cryptography is also a book about cryptography, but it's a book about sullied cryptography. Our goal is to explicitly describe the real-world constraints and realities of cryptography, and to talk about how to engineer secure cryptographic systems. In some ways, this book is a sequel to Bruce Schneier's first book, Applied Cryptography, which was first published ten years ago. But while Applied Cryptography gives a broad overview of cryptography and the myriad possibilities cryptography can offer, this book is narrow and focused. We don't give you dozens of choices; we give you one option and tell you how to implement it correctly. Applied Cryptography displays the wondrous possibilities of cryptography as a mathematical science---what is possible and what is attainable; Practical Cryptography gives concrete advice to people who design and implement cryptographic systems.

Practical Cryptography is our attempt to bridge the gap between the promise of cryptography and the reality of cryptography. It's our attempt to teach engineers how to use cryptography to increase security.

We're qualified to write this book because we're both seasoned cryptographers. Bruce is well known from his books Applied Cryptography and Secrets and Lies, and from his newsletter "Crypto-Gram." Niels Ferguson cut his cryptographic teeth building cryptographic payment systems at the CWI (Dutch National Research Institute for Mathematics and Computer Science) in Amsterdam, and later at a Dutch company called DigiCash. Bruce designed the Blowfish encryption algorithm, and both of us were on the team that designed Twofish. Niels's research led to the first example of the current generation of efficient anonymous payment protocols. Our combined list of academic papers runs into three digits.

More importantly, we both have extensive experience in designing and building cryptographic systems. From 1991 to 1999, Bruce's consulting company Counterpane Systems provided design and analysis services to some of the largest computer and financial companies in the world. More recently, Counterpane Internet Security, Inc., has provided Managed Security Monitoring services to large corporations and government agencies worldwide. Niels also worked at Counterpane before founding his own consulting company, MacFergus. We've seen cryptography as it lives and breathes in the real world, as it flounders against the realities of engineering or even worse, against the realities of business. We're qualified to write this book because we've had to write it again and again for our consulting clients.

How to Read this Book

Practical Cryptography is more a narrative than a reference. It follows the design of a cryptographic system from the specific algorithm choices, outwards through concentric rings to the infrastructure required to make it work. We discuss a single cryptographic problem---one of establishing a means for two people to communicate securely---that's at the heart of almost every cryptographic application. By focusing on one problem and one design philosophy for solving that problem, it is our belief that we can teach more about the realities of cryptographic engineering.

We've both published books before, and we know that publishing is an imperfect science. Try as we might, this book will not be error-free. We're sorry, but it's simply the way things are. (Oddly enough, cryptographic systems have the same problem; we'll talk about that in a few chapters.) While we've endeavored to make this book as perfect as possible, we have a procedure for ensuring that the inevitable errors get corrected.

We think cryptography is just about the most fun you can have with mathematics. We've tried to imbue this book with that feeling of fun, and we hope you enjoy the results. Thanks for coming along on our ride.

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