In VANETs, better communication efficiency can be achieved by sacrificing security and vice versa. But VANETs cannot get started without either of them. In this paper, we propose a set of mechanisms that can actually reconcile these two contradictory requirements. The main idea is to use message aggregation and group communication. The first class of solutions is based on asymmetric cryptographic primitives, the second class uses symmetric ones, and the third one mixes the two. We have also evaluated the performance potential of one technique and arrived at the conclusion that aggregation in VANETs increases not only efficiency but also security.