Communicating Securely in an Insecure Medium
It was a dark and stormy night. Somewhere in the distance a dog howled. A shiny object caught Alice's eye. A diamond cufflink! Only one person in the household could afford diamond cufflinks! So it was the butler, after all! Alice had to warn Bob. But how could she get a message to him without alerting the butler? If she phoned Bob, the butler might listen on an extension. If she sent a carrier pigeon out the window with the message taped to its foot, how would Bob know it was Alice that was sending the message and not Trudy attempting to frame the butler because he spurned her advances?
That's what this book is about. Not much character development for Alice and Bob, we're afraid; nor do we really get to know the butler. But we do discuss how to communicate securely over an insecure medium.
What do we mean by communicating securely? Alice should be able to send a message to Bob that only Bob can understand, even though Alice can't avoid having others see what she sends. When Bob receives a message, he should be able to know for certain that it was Alice who sent the message, and that nobody tampered with the contents of the message in the time between when Alice launched the message and Bob received it.
What do we mean by an insecure medium? Well, in some dictionary or another, under the definition of "insecure medium" should be a picture of the Internet. The world is evolving towards interconnecting every computer, and people talk about connecting household appliances as well, all into some wonderful global internetwork. How wonderful! You'd be able to send electronic mail to anyone in the world. You'd also be able to control your nuclear power plant with simple commands sent across the network while you were vacationing in Fiji. Or sunny Libya. Or historic Iraq. Inside the network the world is scary. There are links that eavesdroppers can listen in on. Information needs to be forwarded through packet switches, and these switches can be reprogrammed to listen to or modify data in transit.
The situation might seem hopeless, but we may yet be saved by the magic of mathematics, and in particular cryptography, which can take a message and transform it into a bunch of numbers known as ciphertext. The ciphertext is unintelligible gibberish except to someone who knows the secret to reversing the transformation. Cryptography allows us to disguise our data so that eavesdroppers gain no information from listening to the information as transmitted. Cryptography also allows us to create an unforgeable message and detect if it has been modified in transit. One method of accomplishing this is with a digital signature, a number associated with a message and its sender that can be verified as authentic by others, but can only be generated by the sender. This should seem astonishing. How can there be a number which you can verify but not generate? A person's handwritten signature can (more or less) only be generated by that person, though it can be verified by others. But it would seem as if a number shouldn't be hard to generate, especially if it can be verified. Theoretically, you could generate someone's signature by trying lots of numbers and testing each one until one passed the verification test. But with the size of the numbers used, it would take too much compute time (for instance, several universe lifetimes) to generate the signature that way. So a digital signature has the same property as a handwritten signature, in that it can only be generated by one person. But a digital signature does more than a handwritten signature. Since the digital signature depends on the contents of the message, if someone alters the message the signature will no longer be correct and the tampering will be detected. This will all become clear if you read Chapter 2 Introduction to Cryptography.
Cryptography is a major theme in this book, not because cryptography is intrinsically interesting (which it is), but because many of the security features people want in a computer network can best be provided through cryptography.
1.1 Roadmap to the Book
After this introductory chapter, there are five main sections in the book:
Part 1 CRYPTOGRAPHY Chapter 2 Introduction to Cryptography is the only part of the cryptography section of the book essential for understanding the rest of the book, since it explains the generic properties of secret key, message digest, and public key algorithms, and how each is used. We've tried our best to make the descriptions of the actual cryptographic algorithms nonthreatening yet thorough, and to give intuition into why they work. It's intended to be readable by anyone, not just graduate students in mathematics. Never once do we use the term lemma. We do hope you read Chapter 3 Secret Key Cryptography, Chapter 4 Modes of Operation, Chapter 5 Hashes and Message Digests, and Chapter 6 Public Key Algorithms which give the details of the popular standards, but it's also OK to skip them and save them for later, or just for reference. Chapter 7 Number Theory and Chapter 8 Math with AES and Elliptic Curves gives a deeper treatment of the mathematics behind the cryptography. Reading them is not necessary for understanding the rest of the book.
Part 2 AUTHENTICATION Chapter 9 Overview of Authentication Systems introduces the general issues involved in proving your identity across a network. Chapter 10 Authentication of People deals with the special circumstances when the device proving its identity is a ω human being. Chapter 11 Security Handshake Pitfalls deals with the details of authentication handshakes. There are many security flaws that keep getting designed into protocols. This chapter attempts to describe variations of authentication handshakes and their relative security and performance strengths. We end the chapter with a checklist of security attacks so that someone designing a protocol can specifically check their protocol for these flaws.
Part 3 STANDARDS This portion of the book describes the standards: Kerberos versions 4 and 5, certificate and PKI standards, IPsec, and SSL. We hope that our descriptions will be much more readable than the standards themselves. And aside from just describing the standards, we give intuition behind the various choices, and criticisms where they are overly complex or have flaws. We hope that our commentary will make the descriptions more interesting and provide a deeper understanding of the design decisions. Our descriptions are not meant to, and cannot, replace reading the standards themselves, since the standards are subject to change. But we hope that after reading our description, it will be much easier to understand the standards.
Part 4 ELECTRONIC MAIL Chapter 20 Electronic Mail Security describes the various types of security features one might want, and how they might be provided. Chapter 21 PEM & S/MIME and Chapter 22 PGP (Pretty Good Privacy) describe the specifics of PEM, S/MIME, and PGP.
Part 5 LEFTOVERS Chapter 23 Firewalls talks about what firewalls are, what problems they solve, and what problems they do not solve. Chapter 24 More Security Systems, describes a variety of security systems, including Novell NetWare (Versions 3 and 4), Lotus Notes, DCE, KryptoKnight/NetSP, Clipper, SNMP, DASS/SPX, Microsoft (LAN Manager and Windows NT), and sabotage-proof routing protocols. Chapter 25 Web Issues talks about the protocols involved in web surfing: URLs, HTTP, HTML, cookies, etc., and the security issues these raise. We close with Chapter 26 Folklore, which describes the reasoning behind some of the advice you will hear from cryptographers.