The proliferation of Body Sensor Networks (BSNs) has revolutionized healthcare monitoring, but it also introduces security challenges, particularly in roaming scenarios. This article presents a novel Composable Authentication Protocol (CAP) designed to ensure secure roaming in Cloud-assisted Body Sensor Networks. CAP leverages a dynamic, context-aware approach to authentication, adapting to diverse environments and scenarios while guaranteeing data privacy and integrity. We explore CAP's architecture and demonstrate its effectiveness in safeguarding BSNs during roaming, thereby enhancing the security and reliability of healthcare monitoring.