Resistance Strategies: Symmetric Encryption

This chapter provides an introduction to encryption as a resistance strategy. The focus is on symmetric encryption. Steganography and asymmetric encryption are covered in enough detail to demonstrate what symmetric cryptography is not, even though both could be subjects of books in their own right. Likewise, information theory is introduced at the appropriate level to support the discussion on cryptography. The chapter assumes no previous knowledge, and attempts to provide a basis for understanding by beginning with definitions for primitive cryptographic terms and discussing what encryption can and cannot do. To give the reader a feel for how cryptography works, historic examples are introduced and the workings of the ciphers are described in some detail. Although these ciphers are not of practical importance, they are simple enough that the reader can grasp their workings without the extensive math background needed for modern ciphers. The historic ciphers covered include several substitution and transposition ciphers, starting with the simple Caesar cipher. Modern encryption is discussed in the context of its primary uses: block ciphers, stream ciphers, disk encryption, and file encryption. Asymmetric encryption is introduced mostly for its utility in key management and distribution of symmetric keys. Host identification, more properly a topic for Chapter 7, is included in this chapter as a motivating example and technical example of these concepts; particularly to this end, the working of the transport layer security (TLS) stack is described.