ABSTRACT
In this chapter, we will require increasingly more number theoretic results. Appendix C on page 212 is provided as a quick reminder and finger-tip reference for the reader. We begin this section with a formalization of certain terms; some of which we have been using in Chapter 1. First of all, an entity is any person, such as Bob, Alice, Eve, or Mallory introduced in Chapter 1, or a thing, such as a computer terminal, which sends, receives, or manipulates information. We have used the term communication channel, which we now define to be any means of communicating information from one entity to another. If two entities, such as Alice and Bob, are communicating and a third entity, such as Eve or Mallory, tries to interfere, passively or actively, with the data over the communication channel, then that third entity is called an adversary or opponent. A secure channel is one that is not physically accessible to an adversary. An unsecured channel is one from which entities other than those for whom the information was intended, can delete, insert, read, delay, or reorder data. A cryptosystem is said to be secure (against eavesdropping) if an adversary, such as Eve, who eavesdrops on a channel which is sending cryptograms, gains nothing over an entity which does not listen to the communication channel.
