Body Sensor Networks can be used to continuously monitor patients' health. However, secure association of sensors with the patient and key management for providing integrity and confidentiality to the sensor readings are essential. We propose a secure discovery protocol based on the synchronised LED blinking pattern, to enable healthcare workers to authorise the sensor-to-patient association. We also propose a novel key distribution and management scheme that uses keychains to establish group keys for body sensor networks and caters for group key update and re-keying to adapt to membership changes. These protocols have been implemented to demonstrate their feasibility and an initial performance evaluation is presented.