ABSTRACT
This chapter introduces information theory and its use in analyzing simple ciphers. See Denning [36] for another view of much of the material in this chapter. This subject was created by Shannon [106] to give a theoretical foundation for communication and, in particular, for cryptography. He measured the secrecy of a cipher by the uncertainty in the plaintext given the ciphertext. The most secret ciphers are the ones for which an eavesdropper learns nothing at all about the plaintext by seeing the ciphertext. Most ciphers leave some information about the plaintext in the ciphertext. If an eavesdropper has enough ciphertext, he may obtain enough information to break the cipher, at least theoretically. Many ciphers can be broken from just a hundred or so bits of ciphertext. These ciphers are not necessarily insecure, because an enormous computation might be required to break them, and the cryptanalyst might not have enough resources to do it.
