## Applied Cryptography by Bruce Schneier

This is the next entry in the series of Russ reading books that he bought years ago and never got around to reading. Thankfully, this time, the book has aged somewhat better.

This review is for the second edition of Applied Cryptography, published in 1996. Given how important computer security has become, and how central cryptography is to computer security, one might think that the passage of 17 years would make a book effectively obsolete. This turns out not to be the case. Yes, Rijndael (the current AES standard and the most widely-used block cipher), Camellia (the up-and-comer in the block cipher world), and the SHA-2 hash postdate this book and aren’t discussed. Yes, there have been some further developments in elliptic-curve public-key cryptography. And yes, much of the political information in this book, as well as the patent situation for public-key cryptosystems, is now mostly of historical interest. But a surprising amount of this book still applies directly.

Partly that’s because much of Applied Cryptography is focused on general principles, classes of algorithms, and cryptographic protocols, and those do not change quickly. A rather amazing number of techniques and protocols still in widespread use today originated in the 1970s. Block ciphers, stream ciphers, cipher modes, public-key systems, signature systems, hash functions, key exchange, secret splitting, key management, and other, similar topics have not changed substantially. And even in the area of specific algorithms, DES is still in use (unfortunately) and its analysis is valuable for understanding any block cipher. RC4, Triple DES, CAST, RSA, MD4, MD5, and SHA-1 are still part of the cryptography landscape, and Schneier was already warning against using MD5 in 1996. The source code included in this book has, thankfully, been made obsolete by widely-available, high-quality free implementations of all common ciphers, available for any modern operating system. But the math, the explanations, and much of the cryptanalysis is still quite relevant.

While it contains a comprehensive introduction to cryptographic concepts, Applied Cryptography is structured more like a reference manual than a tutorial. The first two chapters provide some foundations. Then, a wide variety of protocols, from the common to the completely esoteric, are discussed across the subsequent four chapters. There are four chapters on the basics of ciphers, another chapter on the mathematics, and then the longest part of the book: twelve chapters that provide a tour of all major and several interesting minor protocols that existed in 1996. These are organized into block ciphers, stream ciphers (and pseudo-random number generators), hash functions, and public-key systems. An entire chapter is devoted to the history and analysis of DES, and I think that was my favorite chapter of the book. Not only is it worth understanding how DES works, but it also provides a comprehensive introduction to the complexities of block cipher design, the politics of interactions with the NSA, and a fascinating history of early computer cryptosystems.

Finally, Applied Cryptography closes with chapters on real-world systems and on politics. The politics section is mostly a historical curiosity, as is much of the chapter on real-world systems. Schneier discusses some of the PKCS and X.509 protocols here but doesn’t use SSL as one of his examples despite its specification predating this book (perhaps the most glaring omission of the book). But he does discuss Kerberos (which is of personal interest to me), and it was useful to see it analyzed in the broader context of this sort of book.

Those who read Schneier’s blog regularly, or who have read any of his other books, will know that he’s concise, incisive, and very readable, even with difficult material. I think he does err on the side of compactness—this is not a book that handholds the reader or explains something more than once—but there’s a lot of ground to cover, so I can’t really blame him. Expect to have to read the more difficult parts several times, and (at least on a first read) expect to just skip past some of the most complex sections as probably not worth the effort. But I think Schneier does a great job structuring the book so that one comes away with a high-level impression and overview sufficient to make informed guesses about capabilities and relative difficulty even if one doesn’t have the patience to work through the details.

I suspect most readers will want to skim at least part of this book. Unless you’re willing to do a lot of background reading to understand the math, or a lot of careful study to follow the detailed descriptions and flaws in often-obscure algorithms, the algorithm discussions start to blend together. I found myself skipping a lot of the math and focusing on the basic descriptions and the cryptanalysis, particularly for the obscure ciphers. Many of the algorithms are also highly opaque and of obscure benefit; I doubt I will ever care about blind signatures or oblivious transfer, and I struggled to wrap my mind around the details of bit commitment (although it’s apparently important in zero knowledge proofs, which in turn are relevant to authentication). But this is not the sort of book where you have to read every word. It’s very well-structured, provides clear resynchronization points for readers who have had enough of a particular topic, and marks the topics that are more obscure or of dubious general usefulness.

The most useful parts of this book, in my opinion, are the basic conceptual introductions to each class of cryptographic algorithm or protocol, and those chapters are excellent. I’ve been working in applied computer security, at the application and deployment level, for years, and this is the first time that I’ve felt like I really understood the difference between a block cipher and a stream cipher, or the implications of block cipher mode selection. Applied Cryptography is also a fascinating exercise in adjusting one’s intuition for where the complexity lies. It’s a memorable experience to move from the chapters on block ciphers, full of S-boxes and complex mixing and magic numbers, into the far more straightforward mathematical analysis of stream ciphers. Or the mathematical simplicity of RSA (although quite a bit of complexity is lurking on the cryptanalysis side, much of which is only touched on here).

Of more historical interest, but still quite impressive, is that Applied Cryptography doubles as a comprehensive literature review. Not only does it cover nearly every algorithm in use at the time, it discusses the cryptography literature on nearly every topic with brief summaries of results and lines of investigation. The references section is a stunning 66 pages of small print featuring 1,653 references, and those are mentioned and put in context throughout the book. I’m not personally interested in chasing those lines of research further, and of course 17 years means there are many new papers that are as important to read today, but it’s obvious that this book would have been an amazing and invaluable map to the research territory when it was first published.

There are now newer books on this topic that you should consider if you’re purchasing a book today. Cryptography Engineering in particular is in my to-read pile, and I’m interested to see how much of this book it could replace, although I believe it’s lighter on the mathematical analysis and details. But I think Applied Cryptography still has a place on the bookshelf of any computer security professional. Schneier is succinct, detailed, straightforward, and comprehensive, at least for 1996, and I know I’ll be reaching for this book the next time I forget the details of CFB mode and then get hopelessly confused by the Wikipedia article on the topic.

Categories: Applied Cryptography, Text