Primal infon logic (PIL), introduced by Gurevich and Neeman in 2009, is a logic for authorization in distributed systems. It is a variant of the (and, implies)-fragment of intuitionistic modal logic. It presents many interesting technical challenges -- one of them is to determine the complexity of the derivability problem. Previously, some restrictions of propositional PIL were proved to have a linear time algorithm, and some extensions have been proved to be PSPACE-complete. In this paper, we provide an O(N^3) algorithm for derivability in propositional PIL. The solution involves an interesting interplay between the sequent calculus formulation (to prove the subformula property) and the natural deduction formulation of the logic (based on which we provide an algorithm for the derivability problem).
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