ABSTRACT
Recall from Chapter 3 that some cryptosystems seem to do a better job of “hiding” the plaintext and some cryptosystems are harder to cryptanalyse than others. In order to formalize these notions, and to be able to provide a formal measurement of the usefulness of a cryptosystem, it is necessary to have a basic grounding in information theory. This is the topic of this chapter. It will include:
• Entropy, or the formal quantification of uncertainty. • Methods of quantifying the secrecy of a cryptosystem. • The entropy and redundancy of English. • The unicity distance, which is the amount of ciphertext required to obtain a unique plaintext.
