Security Engineering, 3rd Edition

CHAPTER 5Cryptography

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– G JULIUS CAESAR

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– JOHN F KENNEDY

5.1 Introduction

Cryptography is where security engineering meets mathematics. It gives us the tools that underlie most modern security protocols. It is the key technology for protecting distributed systems, yet it is surprisingly hard to do right. As we've already seen in Chapter 4, “Protocols,” cryptography has often been used to protect the wrong things, or to protect them in the wrong way. Unfortunately, the available crypto tools aren't always very usable.

But no security engineer can ignore cryptology. A medical friend once told me that while she was young, she worked overseas in a country where, for economic reasons, they'd shortened their medical degrees and concentrated on producing specialists as quickly as possible. One day, a patient who'd had both kidneys removed and was awaiting a transplant needed her dialysis shunt redone. The surgeon sent the patient back from the theater on the grounds that there was no urinalysis on file. It just didn't occur to him that a patient with no kidneys couldn't produce any urine.

Just as a doctor needs to understand physiology as well as surgery, so a security engineer needs to be familiar with at least the basics of crypto (and much else). There are, broadly speaking, three levels at which one can approach crypto. The first consists ...