AbstractFormal methods have been proved successful in analyzing different kinds of security protocols. They typically formalize and study the security guarantees provided by cryptographic protocols, when executed by a (possibly unbounded) number of different participants. A key problem in applying formal methods to cryptographic protocols, is the study of multi-protocol systems, where different protocols are concurrently executed. This scenario is particularly interesting in a global computing setting, where several different security services coexist and are possibly combined together. In this paper, we discuss how the tagging mechanism presented in [M. Bugliesi, R. Focardi, and M. Maffei. Compositional analysis of authentication protocols. In Proceedings of European Symposium on Programming (ESOP 2004), volume 2986 of Lecture Notes in Computer Science, pages 140–154. Springer-Verlag, 2004, M. Bugliesi, R.Focardi, and M.Maffei. A theory of types and effects for authentication. In ACM Proceedings of Formal Methods for Security Engineering: from Theory to Practice (FMSE 2004), pages 1–12. ACM Press, October 2004] addresses this issue.