Cryptographic protocol verification using tractable classes of horn clauses

We consider secrecy problems for cryptographic protocols modeled using Horn clauses and present general classes of Horn clauses which can be efficiently decided. Besides simplifying the methods for the class of flat and onevariable clauses introduced for modeling of protocols with single blind copying [7,25], we also generalize this class by considering k-variable clauses instead of one-variable clauses with suitable restrictions similar to those for the class S(+). This class allows to conveniently model protocols with joint blind copying. We show that for a fixed k, our new class can be decided in DEXPTIME, as in the case of one variable.