Privacy properties such as untraceability, vote secrecy, or anonymity are typically expressed as behavioural equivalence in a process algebra that models security protocols. In this paper, we study how to decide one particular relation, namely trace equivalence, for an unbounded number of sessions. 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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