We identify the security challenges facing a sensor network for wireless health monitoring, and propose an architecture called "SNAP" (Sensor Network for Assessment of Patients) to address these challenges. SNAP protects the privacy, authenticity, and integrity of medical data, with low-cost energy-efficient mechanisms. We have incorporated the following mechanisms in SNAP: (1) an ECC-based secure key exchange protocol to set up shared keys between sensor nodes and base stations; (2) symmetric encryption and decryption for protecting data confidentiality and integrity; (3) a two-tier authentication scheme for verifying data source. We have developed a prototype on the Tmote Sky platform for evaluating the proposed architecture and mechanisms. Categories and Subject Descriptors C.2.0 [Computer-Communication Networks]: General-security and protection; J.3 [Computer Applications]: Life and Medical Sciences--medical information systems General Terms Security, Human Factors Keywo...