A measure of semiequivocation

Abstract

A Shannon-theoretic cryptographic model is described in which the purpose of the cryptanalist is to find a set of M elements containing the solution, rather than finding the solution itself. For M = 2 we introduce the notions of semientropy, semiequivocation and duplicity distance, which are counterparts to well-known notions met in the case M = 1. It is argued that in some situation our model takes into account the semantical competence of the cryptanalist. (as opposed to his statistical competence) better than the usual model does.