We propose a set of security provisions for node to base station communication in wireless sensor networks. It supports standard security requirements, viz. authentication of the origin of data and confidentiality of data. Additionally we use key evolution to achieve forward security which is of particular importance in the face of node capture attacks. As a bonus we obtain implicit weak freshness without message expansion. We take the typical resource constraints of wireless sensor networks into account. The security provisions can be superimposed on several communication models, such as the epidemic communication model.