Noname manuscript No. (will be inserted by the editor) Composition of Password-based Protocols

Abstract Formal and symbolic techniques are extremely useful for modelling and analysing security protocols. They have helped to improve our understanding of such protocols, allowed us to discover flaws, and they also provide support for protocol design. However, such analyses usually consider that the protocol is ex-ecuted in isolation or assume a bounded number of protocol sessions. Hence, no security guarantee is provided when the protocol is executed in a more complex environment. In this paper, we study whether password protocols can be safely composed, even when a same password is reused. More precisely, we present a transformation which maps a password protocol that is secure for a single protocol session (a de-cidable problem) to a protocol that is secure for an unbounded number of sessions. Our result provides an effective strategy to design secure password protocols: (i) design a protocol intended to be secure for one protocol session; (ii) apply our transformation and obtain a protocol which is secure for an unbounded numbe