Equivocations for homophonic ciphers

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Abstract

Substitution ciphers can be quite weak when the probability distribution of the message letters is distinctly non-uniform. A time-honoured solution to remove this weakness is to “split” each high-probability letter into a number of “homophones” and use a substitution cipher for the resulting extended alphabet. Here the performance of a homophonic cipher is studied from a Shannon-theoretic point of view. The key and message equivocations (conditional entropies given the intercepted cryptogram) are computed both for finite-length messages and “very long” messages. The results obtained are strictly related to those found by Blom and Dunham for substitution ciphers. The key space of a homophonic cipher is specified carefully, so as to avoid misunderstandings which appear to have occurred on this subject.

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APA

Sgarro, A. (1985). Equivocations for homophonic ciphers. In Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) (Vol. 209 LNCS, pp. 51–61). Springer Verlag. https://doi.org/10.1007/3-540-39757-4_6

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