d Abstract) Stefan Lucks1 , Erik Zenner2 , Andr´e Weimerskirch3 , Dirk Westhoff4 1 Theoretische Informatik, University of Mannheim, Germany 2 Erik Zenner, Cryptico A/S, Copenhagen, Danmark 3 escrypt - Embedded Security GmbH, Bochum, Germany 4 NEC Heidelberg, Germany : Message authenticity (knowing “who sent this message”) is an important security issue for sensor networks, and often difficult to solve. Sometimes, it may be sufficient and more efficient to solve the simpler entitiy recognition problem, instead: “is the message from the same entity that sent the previous messages?”. This paper describes entity recognition for sensor network motes. A protocol presented at SAC 2003 [5] is shown to be insecure, and a new and provably secure protocol is proposed.