Introduction to Network Security
- 1.1 Opinions, Products
- 1.2 Roadmap to the Book
- 1.3 Terminology
- 1.4 Notation
- 1.5 Cryptographically Protected Sessions
- 1.6 Active and Passive Attacks
- 1.7 Legal Issues
- 1.8 Some Network Basics
- 1.9 Names for Humans
- 1.10 Authentication and Authorization
- 1.11 Malware: Viruses, Worms, Trojan Horses
- 1.12 Security Gateway
- 1.13 Denial-of-Service (DoS) Attacks
- 1.14 NAT (Network Address Translation)
The authors discuss how to communicate securely over an insecure medium through cryptography.
This chapter is from the book
This chapter is from the book
This chapter is from the book
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, household appliance, automobile, child’s toy, and embedded medical device, all into some wonderful global internetwork. How wonderful! You’d be able to control your nuclear power plant with simple commands sent across the network while you were vacationing in Fiji. Or sunny Havana. Or historic Pyongyang. 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 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 you can verify but not generate? A person’s handwritten signature can (more or less) only be generated by that person, though it can (more or less) 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 digital 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 (theoretically) has, in that it can only be generated by one person but can be verified by lots of people. 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 Opinions, Products
Opinions expressed are those of the authors alone (and possibly not even agreed upon by all the authors). Opinions do not necessarily represent the views of any of the authors’ past, current, or future employers. Any mention of commercial products or reference to commercial organizations is for information only. It does not imply recommendation or endorsement by NIST or any of the current, future, or prior organizations employing any of the authors.