In this chapter, we first introduce the security services we typically expect in a network. We then show how these services can be provided using cryptography. At the end of the chapter, we also touch on the issue of distributing symmetric and asymmetric keys. | Chapter 30 Cryptography Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30. 30-1 INTRODUCTION Let us introduce the issues involved in cryptography. First, we need to define some terms; then we give some taxonomies. Definitions Two Categories Topics discussed in this section: 30. Figure Cryptography components 30. Figure Categories of cryptography 30. Figure Symmetric-key cryptography 30. In symmetric-key cryptography, the same key is used by the sender (for encryption) and the receiver (for decryption). The key is shared. Note 30. Figure Asymmetric-key cryptography 30. Figure Keys used in cryptography 30. Figure Comparison between two categories of cryptography 30. 30-2 SYMMETRIC-KEY CRYPTOGRAPHY Symmetric-key cryptography started thousands of years ago when people needed to exchange secrets (for example, in a war). We still mainly use symmetric-key cryptography in our network security. Traditional Ciphers Simple Modern Ciphers Modern Round Ciphers Mode of Operation Topics discussed in this section: 30. Figure Traditional ciphers 30. A substitution cipher replaces one symbol with another. Note 30. The following shows a plaintext and its corresponding ciphertext. Is the cipher monoalphabetic? Example Solution The cipher is probably monoalphabetic because both occurrences of L’s are encrypted as O’s. 30. The following shows a plaintext and its corresponding ciphertext. Is the cipher monoalphabetic? Example Solution The cipher is not monoalphabetic because each occurrence of L is encrypted by a different character. The first L is encrypted as N; the second as Z. 30. The shift cipher is sometimes referred to as the Caesar cipher. Note 30. Use the shift cipher with key = 15 to encrypt the message “HELLO.” Solution We encrypt one character at a time. Each character is shifted 15 characters down. Letter H is encrypted to W. | Chapter 30 Cryptography Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30. 30-1 INTRODUCTION Let us introduce the issues involved in cryptography. First, we need to define some terms; then we give some taxonomies. Definitions Two Categories Topics discussed in this section: 30. Figure Cryptography components 30. Figure Categories of cryptography 30. Figure Symmetric-key cryptography 30. In symmetric-key cryptography, the same key is used by the sender (for encryption) and the receiver (for decryption). The key is shared. Note 30. Figure Asymmetric-key cryptography 30. Figure Keys used in cryptography 30. Figure Comparison between two categories of cryptography 30. 30-2 SYMMETRIC-KEY CRYPTOGRAPHY Symmetric-key cryptography started thousands of years ago when people needed to exchange secrets (for example, in a war). We still mainly use symmetric-key cryptography in our .
