Wireless body sensor networks (BSN) are being used to continuously monitor the patient's conditions and recovery progress. It is very important to secure the confidentiality, integrity and authenticity of the patient's health record in such applications. In this paper, we propose a novel key distribution and management scheme that uses keychains to establish group keys for body sensor networks. This scheme caters for efficient group key update and re-keying in order to adapt to membership changes. We also present a lightweight approach to enable sensor-to-sensor authentication in the BSNs. Sensors authenticate each other by computing a Elliptic Curve Diffie-Hellman (ECDH) key between each other based on an authenticated membership broadcast received from the patient's device. Both protocols have been implemented on Tmote Sky platform, analysed and evaluated to demonstrate their feasibility. We also shows the security analysis of the protocols using BAN Logic.