Chapter 10. Cryptography Basics

This chapter explains the basics of cryptography on which many secure Internet protocols are based. This chapter also explores the ways in which the use of cryptography is regulated by politics and U.S. law. Chapter 11, explores the specific ways in which cryptography is used today on the World Wide Web.

Understanding Cryptography

Cryptography is a collection of techniques for keeping information secure. Using cryptography, you can transform written words and other kinds of messages so that they are unintelligible to unauthorized recipients. An authorized recipient can then transform the words or messages back into a message that is perfectly understandable.

For example, here is a message that you might want to encrypt:

SSL is a cryptographic protocol

And here is the message after it has been encrypted:


Even better, with cryptography you can transform this gibberish back into the original easily understood message.

Roots of Cryptography

The idea of cryptography is thousands of years old: Greek and Roman generals used cryptography to send coded messages to commanders who were in the field. Those early systems were based on two techniques: substitution and transposition.

Substitution is based on the principle of replacing each letter in the message you wish to encrypt with another one. The Caesar cipher, for example, substitutes the letter “a” with the letter “d,” the letter “b” with the letter “e,” and so on. ...

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