Cryptanalytical finite automaton invertibility with finite delay | Prikladnaya Diskretnaya Matematika - Applied Discrete Mathematics. 2019. № 46. DOI: 10.17223/20710410/46/3

The paper continues an investigation of the cryptanalytical invertibility concept with a finite delay introduced by the author for finite automata. Here, we expound an algorithmic test for an automaton A to be cryptanalytically invertible with a finite delay, that is, to have a recovering function f which allows to calculate a prefix of a length m in an input sequence of the automaton A by using its output sequence of a length m + τ and some additional information about A defining a type of its invertibility and known to cryptanalysts. The test finds out whether the automaton A has a recovering function f or not and if it has, determines some or, may be, all of such functions. The test algorithm simulates a backtracking method for searching a possibility to transform a binary relation to a function by shortening its domain to a set corresponding to the invertibility type under consideration.