Our first main contribution is to reduce the search space for attacks. Specifically, we show that if there is an attack then there is one that is well-typed. Our result holds for a large class of typing systems and a large class of determinate security protocols. Assuming finitely many nonces and keys, we can derive from this result that trace equivalence is decidable for an unbounded number of sessions for a class of tagged protocols, yielding one of the first decidability results for the unbounded case. As an intermediate result, we also provide a novel decision procedure in the case of a bounded number of sessions.
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