Communication Theory of Secrecy Systems by C. At the. Shanon Article

Conversation Theory of Secrecy Devices by C. E. Shanon

Communication Theory of Secrecy Systems?



The difficulties of cryptography and secrecy systems supply an interesting using communication theory1. In this paper a theory of secrecy systems is usually developed. The approach is definitely on a theoretical level and it is intended to complement the treatment seen in standard ideal for cryptography2. Right now there, a detailed analyze is made of the countless standard types of unique codes and ciphers, and of many ways of breaking them. We will be more concerned with the general numerical structure and properties of secrecy systems.

The treatment is limited in certain techniques. First, you will discover three general types of secrecy system: (1) concealment systems, which include such methods as invisible ink, concealing a message in an innocent textual content, or in a false covering cryptogram, or different methods where the existence of the message is usually concealed in the enemy; (2) privacy systems, for example conversation inversion, through which special machines are required to restore the communication; (3) " true” secrecy systems where the meaning from the message is usually concealed simply by cipher, code, etc ., although its lifestyle is not hidden, plus the enemy is definitely assumed to obtain any particular equipment important to intercept and record the transmitted transmission. We consider only the third type—concealment system are primarily a mental problem and privacy systems a technological one.

Second, the treatment is restricted to the case of discrete information where message to be enciphered consists of a sequence of discrete icons, each selected from a finite established. These signs may be albhabets in a dialect, words of any language, extravagance levels of a " quantized” speech or video sign, etc ., but the main emphasis and thinking has been interested in the case of letters.

The paper can be divided into three parts. The key results will be in short , summarized. The first component deals with the basic mathematical structure of secrecy systems. Just as communication theory a dialect is considered to be symbolized by a stochastic process which produces a discrete sequence of? The material in this paper made an appearance in a private report " A Mathematical Theory of Cryptography” went out with Sept. one particular, 1946, that has now recently been declassified.

you Shannon, C. E., " A Statistical Theory of Communication, ” Bell Program Technical Journal, July 1948, p. 623.

2 Find, for example , They would. F. Gaines, " Primary Cryptanalysis, ” or Meters. Givierge, " Cours para Cryptographie. ” symbols according to some system of probabilities. Connected with a language there is a certain parameter G which we all call the redundancy from the language. G measures, in a way, how much a text in the language can be reduced long without losing any information. As a basic example, as u often follows queen in English language words, the u might be omitted without loss. Substantial reductions are possible in English as a result of statistical framework of the terminology, the highs of selected letters or perhaps words, etc . Redundancy is of central importance in the analyze of secrecy systems.

A secrecy system is defined abstractly as a pair of transformations of 1 space (the set of conceivable messages) into a second space (the pair of possible cryptograms). Each particular transformation with the set compares to enciphering having a particular key. The conversions are intended reversible (non-singular) so that exceptional deciphering is achievable when the key is known.

Every single key and for that reason each modification is presumed to have an dialectic probability linked to it—the possibility of choosing that key. In the same way each possible message is definitely assumed with an associated dialectic probability, determined by the fundamental stochastic procedure. These possibilities for the many keys and messages are in reality the enemy cryptanalyst's von vornherein probabilities pertaining to the choices under consideration, and represent his dialectic knowledge of the specific situation....

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