Simple Substitution Ciphers
Substitution ciphers are a rather unsophisticated and time-honoured class of ciphers, to the extent that one may feel that they appeal to aesthetical-minded mathematicians rather than to communications engineers or to computer scientists. However, one should not overlook the fact that individually weak ciphers can be combined into a network to give a hopefully strong cryptographic system: an obvious example here is the Data Encryption Standard, or DES, developed by IBM, which is built up by concatenating substitution and transposition ciphers.
KeywordsError Probability Exponential Term Conditional Entropy Composition Class Data Encryption Standard
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