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gAPTER 29 Cryptography We begin our discussion of network security with an introduction to cryptography and adiscussion of the methods used in security management. The science of cryptography is very complex; there are entire books devoted to the subject. A cryptography expert needs to be knowledgeable in areas such as mathematics, electronics, and program- ming. In this chapter, we consider the concepts needed to understand the security issues discussed in Chapter 30 and network security discussed in Chapter 31. We focus on symmetric-key cryptography, which is presently more common than public-key cryptography. Symmetric-key cryptography is less math-based than public- key cryptography, which has its origins in number theory. Cryptography and its applications to the Internet are a relatively new field whose importance increases with every new attack on the Internet. 29.1 INTRODUCTION The word cryptography in Greek means “secret writing.” However, the term today refers to the science and art of transforming messages to make them secure and immune to attacks. Figure 29.1 shows the components involved in cryptography. Figure 29,1 Cryptography components Sender Receiver = Plaintext | plaintext Encrypti Network = Vee ciphertext eee ciphertext if The original message, before being transformed, is called plaintext. After the “ssage is transformed, it is called ciphertext. An encryption algorithm transforms * plaintext to ciphertext; a decryption algorithm transforms the ciphertext back to 
