Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth)
(Publisher: John Wiley & Sons, Inc.)
Author(s): Bruce Schneier
ISBN: 0471128457
Publication Date: 01/01/96

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Part III
Cryptographic Algorithms

Chapter 11
Mathematical Background

11.1 Information Theory

Modern information theory was first published in 1948 by Claude Elmwood Shannon [1431, 1432]. (His papers have been reprinted by the IEEE Press [1433].) For a good mathematical treatment of the topic, consult [593]. In this section, I will just sketch some important ideas.

Entropy and Uncertainty

Information theory defines the amount of information in a message as the minimum number of bits needed to encode all possible meanings of that message, assuming all messages are equally likely. For example, the day-of-the-week field in a database contains no more than 3 bits of information, because the information can be encoded with 3 bits:

    000 = Sunday
    001 = Monday
    010 = Tuesday
    011 = Wednesday
    100 = Thursday
    101 = Friday
    110 = Saturday
    111 is unused

If this information were represented by corresponding ASCII character strings, it would take up more memory space but would not contain any more information. Similarly, the “sex” field of a database contains only 1 bit of information, even though it might be stored as one of two 6-byte ASCII strings: “MALE” or “FEMALE.”

Formally, the amount of information in a message M is measured by the entropy of a message, denoted by H(M). The entropy of a message indicating sex is 1 bit; the entropy of a message indicating the day of the week is slightly less than 3 bits. In general, the entropy of a message measured in bits is log2 n, in which n is the number of possible meanings. This assumes that each meaning is equally likely.

The entropy of a message also measures its uncertainty. This is the number of plaintext bits needed to be recovered when the message is scrambled in ciphertext in order to learn the plaintext. For example, if the ciphertext block “QHP*5M” is either “MALE” or “FEMALE, ” then the uncertainty of the message is 1. A cryptanalyst has to learn only one well-chosen bit to recover the message.

Rate of a Language

For a given language, the rate of the language is

r = H(M)/N

in which N is the length of the message. The rate of normal English takes various values between 1.0 bits/letter and 1.5 bits/letter, for large values of N. Shannon, in [1434], said that the entropy depends on the length of the text. Specifically he indicated a rate of 2.3 bits/letter for 8-letter chunks, but the rate drops to between 1.3 and 1.5 for 16-letter chunks. Thomas Cover used a gambling estimating technique and found an entropy of 1.3 bits/character [386]. (I’ll use 1.3 in this book.) The absolute rate of a language is the maximum number of bits that can be coded in each character, assuming each character sequence is equally likely. If there are L characters in a language, the absolute rate is:

R = log2 L

This is the maximum entropy of the individual characters.

For English, with 26 letters, the absolute rate is log2 26, or about 4.7 bits/letter. It should come as no surprise to anyone that the actual rate of English is much less than the absolute rate; natural language is highly redundant.

The redundancy of a language, denoted D, is defined by:

D = R - r

Given that the rate of English is 1.3, the redundancy is 3.4 bits/letter. This means that each English character carries 3.4 bits of redundant information.

An ASCII message that is nothing more than printed English has 1.3 bits of information per byte of message. This means it has 6.7 bits of redundant information, giving it an overall redundancy of 0.84 bits of information per bit of ASCII text, and an entropy of 0.16 bits of information per bit of ASCII text. The same message in BAUDOT, at 5 bits per character, has a redundancy of 0.74 bits per bit and an entropy of 0.26 bits per bit. Spacing, punctuation, numbers, and formatting modify these results.

Security of a Cryptosystem

Shannon defined a precise mathematical model of what it means for a cryptosystem to be secure. The goal of a cryptanalyst is to determine the key K, the plaintext P, or both. However, he may be satisfied with some probabilistic information about P: whether it is digitized audio, German text, spreadsheet data, or something else.

In most real-world cryptanalysis, the cryptanalyst has some probabilistic information about P before he even starts. He probably knows the language of the plaintext. This language has a certain redundancy associated with it. If it is a message to Bob, it probably begins with “Dear Bob.” Certainly “Dear Bob” is more probable than “e8T&g [, m.” The purpose of cryptanalysis is to modify the probabilities associated with each possible plaintext. Eventually one plaintext will emerge from the pile of possible plaintexts as certain (or at least, very probable).

There is such a thing as a cryptosystem that achieves perfect secrecy: a cryptosystem in which the ciphertext yields no possible information about the plaintext (except possibly its length). Shannon theorized that it is only possible if the number of possible keys is at least as large as the number of possible messages. In other words, the key must be at least as long as the message itself, and no key can be reused. In still other words, the one-time pad (see Section 1.5) is the only cryptosystem that achieves perfect secrecy.

Perfect secrecy aside, the ciphertext unavoidably yields some information about the corresponding plaintext. A good cryptographic algorithm keeps this information to a minimum; a good cryptanalyst exploits this information to determine the plaintext.

Cryptanalysts use the natural redundancy of language to reduce the number of possible plaintexts. The more redundant the language, the easier it is to cryptanalyze. This is the reason that many real-world cryptographic implementations use a compression program to reduce the size of the text before encrypting it. Compression reduces the redundancy of a message as well as the work required to encrypt and decrypt.

The entropy of a cryptosystem is a measure of the size of the keyspace, K. It is approximated by the base two logarithm of the number of keys:

H(K) = log2 K

A cryptosystem with a 64-bit key has an entropy of 64 bits; a cryptosystem with a 56-bit key has an entropy of 56 bits. In general, the greater the entropy, the harder it is to break a cryptosystem.

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