Private-Key Encryption

Abstract

In this chapter we introduce the basic ideas and concepts underlying private-key encryption (also called symmetric encryption). Particularly important is the concept of security, which for the classical ciphers studied in the first chapter was not precisely defined. Thus we shall start by looking at the first rigorous definition for this concept, namely perfect secrecy which, while providing the strongest security assurance, has important practical disadvantages that motivate the introduction of computational security concepts that are more suited for practical use. These notions are defined with the help of complexity theory and the concept of pseudorandomness, a weakening of randomness that takes into account the fact that computational resources are limited. The security notions also make use of the concepts of one-way function and pseudo-random generator, which are of crucial importance for cryptography and are also introduced here.