Web Security, Privacy & Commerce, 2nd Edition

Chapter 3. Cryptography Basics

This chapter explains the basics of cryptography, on which many secure Internet protocols are based. Cryptography is a complex topic and in this chapter we’re obviously presenting only a summary. Chapter 4 describes how cryptography is used today on the Web. For more complete information on cryptography concepts and algorithms, see the references in Appendix E.

Understanding Cryptography

Cryptography is a collection of mathematical techniques for protecting information. Using cryptography, you can transform written words and other kinds of messages so that they are unintelligible to anyone who does not possess a specific mathematical key necessary to unlock the message. The process of using cryptography to scramble a message is called encryption . The process of unscrambling the message by use of the appropriate key is called decryption . Figure 3-1 illustrates how these two processes fit together.

Figure 3-1. Encryption is a process that uses a key to transform a block of plaintext into an encrypted ciphertext. Decryption is the process that takes an encrypted ciphertext and a decryption key and produces the original plaintext.

Cryptography is used to prevent information from being accessed by an unauthorized recipient. In theory, once a piece of information is encrypted, that information can be accidentally disclosed or intercepted by a third party without ...