Читать книгу Cryptography, Information Theory, and Error-Correction - Aiden A. Bruen - Страница 41

Goals, Discussion In this chapter, we survey some historical ciphers which have been used since antiquity. (They are all symmetric ciphers. Public key ciphers were not invented until the 1970s and are discussed in Chapter 3.) Although the ciphers presented here are obsolete, they still provide good examples of cryptographic procedures. For example the Vigenère cipher, being a block cipher, is a forerunner of modern block ciphers such as Advanced Encryption Standard (AES). From these classical ciphers, we can also learn about various attacks in cryptography. This subject is pursued more fully in Chapter 7.

New, Noteworthy We discuss the Vigenère cipher and show how it can be broken by finding the length of the keyword and then the keyword itself. We explain clearly the simple principles involved without getting bogged down in lengthy formulae. We also give a detailed but accessible description of the famous Enigma system used in World War II both from the mechanical and the mathematical point of view.