Chapter 6. More on Symmetric Ciphers

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6.1 Multiple Encryption and Triple DES

Double DES

Triple DES with Two Keys

Triple DES with Three Keys

6.2 Block Cipher Modes of Operation

Electronic Codebook Mode

Cipher Block Chaining Mode

Cipher Feedback Mode

Output Feedback Mode

Counter Mode

6.3 Stream Ciphers and RC4

Stream Cipher Structure

The RC4 Algorithm

6.4 Recommended Reading and Web Site

6.5 Key Terms, Review Questions, and Problems

Key Terms

Review Questions

Problems

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"I am fairly familiar with all the forms of secret writings, and am myself the author of a trifling monograph upon the subject, in which I analyze one hundred and sixty separate ciphers," said Holmes.

The Adventure of the Dancing Men, Sir Arthur Conan Doyle

Key Points

• Multiple encryption is a technique in which an encryption algorithm is used multiple times. In the first instance, plaintext is converted to ciphertext using the encryption algorithm. This ciphertext is then used as input and the algorithm is applied again. This process may be repeated through any number of stages.

• Triple DES makes use of three stages of the DES algorithm, using a total of two or three distinct keys.

• A mode of operation is a technique for enhancing the effect of a cryptographic algorithm or adapting the algorithm for an application, such as applying a block cipher to a sequence of data blocks or a data stream.

• Five modes of operation have been standardized for use with symmetric block ciphers such as DES and AES: electronic codebook mode, cipher block chaining mode, cipher feedback mode, output feedback mode, and counter mode.

• A stream cipher is a symmetric encryption algorithm in which ciphertext output is produced bit-by-bit or byte-by-byte from a stream of plaintext input. The most widely used such cipher is RC4.

This chapter continues our discussion of symmetric ciphers. We begin with the topic of multiple encryption, looking in particular at the most widely used multiple-encryption scheme: triple DES.

The chapter next turns to the subject of block cipher modes of operation. We find that there are a number of different ways to apply a block cipher to plaintext, each with its own advantages and particular applications.

Finally, this chapter addresses the subject of symmetric stream ciphers, which differ in significant ways from symmetric block ciphers. We also look at the most important such cipher, RC4.