Commit b5a61258 authored by Florent Chehab's avatar Florent Chehab


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\section{Article : \guill{Communication theory of secrecy system}}
Extrait concernant les critères d'évaluation d'un cryptosystème :
5.1 Amount of Secrecy \\
There are some systems that are perfect—the enemy is no better off after intercepting any amount of material than before. Other systems, although giving him some information, do not yield a unique “solution” to intercepted cryptograms. Among the uniquely solvable systems, there are wide variations in the amount of labor required to effect this solution and in the amount of material that must be intercepted to make the solution unique. 8 This system is trivial from the secrecy standpoint since, with the exception of the first d letters, the enemy is in possession of the entire “key”. 669
5.2 Size of Key \\
The key must be transmitted by non-interceptible means from transmitting to receiving points. Sometimes it must be memorized. It is therefore desirable to have the key as small as possible.
5.3 Complexity of Enciphering and Deciphering \\
Operations Enciphering and deciphering should, of course, be as simple as possible. If they are done manually, complexity leads to loss of time, errors, etc. If done mechanically, complexity leads to large expensive machines.
5.4 Propagation of Errors \\
In certain types of ciphers an error of one letter in enciphering or transmission leads to a large number of errors in the deciphered text. The error are spread out by the deciphering operation, causing the loss of much information and frequent need for repetition of the cryptogram. It is naturally desirable to minimize this error expansion.
5.5 Expansion of Message \\
In some types of secrecy systems the size of the message is increased by the enciphering process. This undesirable effect may be seen in systems where one attempts to swamp out message statistics by the addition of many nulls, or where multiple substitutes are used. It also occurs in many “concealment” types of systems (which are not usually secrecy systems in the sense of our definition).
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