We investigate the design space of sensor network broadcast authentication. We show that prior approaches can be organized based on a taxonomy of seven fundamental proprieties, such that each approach can satisfy at most six of the seven proprieties. An empirical study of the design space reveals possibilities of new approaches, which we present in the following two new authentication protocols: RPT and LEA. Based on this taxonomy, we offer guidance in selecting the most appropriate protocol based on an application’s desired proprieties. Finally, we pose the open challenge for the research community to devise a protocol simultaneously providing all seven properties. Categories and Subject Descriptors C.2.0 [Computer-Communication Networks]: General– Security and protection; K.6.5 [Security and Protection]: Security and Protection – Authentication General Terms Security, Design Keywords Broadcast Authentication, Taxnonomy, Sensor Network