ABSTRACT
This chapter introduces notions of cryptographic security, concentrating on the concept of unconditional security. The One-time Pad is presented and concepts such as information theory, entropy, and perfect secrecy are discussed. The chapter discusses several of Shannon’s ideas. It considers some of the various approaches to evaluating the security of a cryptosystem. The chapter presents the concept of entropy, which is used to analyze the unicity distance of a cryptosystem. In practice, people often study the computational security of a cryptosystem with respect to certain specific types of attacks. The unconditional security of a cryptosystem obviously cannot be studied from the point of view of computational complexity because the people allow computation time to be infinite. The historical development of cryptography has been to try to design cryptosystems where one key can be used to encrypt a relatively long string of plaintext and still maintain some measure of computational security.
