Introduction to Cryptography with Open-Source Software

Chapter 4

Introduction to information theory

Recall from Chapter 3 that some cryptosystems see m to do a better job of

“hiding” the plaintext and some cryptosystems are harder to cryptanalyse

than others. In order to fo rmalize 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 ...