To increase the lifespan of wireless sensor networks (WSN) and preserve the energy of sensors, data aggregation techniques are usually used. Aggregation can be seen as the process by which data sent from sensors to the BS are little-by-little processed by some nodes called aggregator nodes. Aggregators collect data from surrounding nodes and produce a small sized output, thus preventing that all nodes in the network send their data to the BS. Security plays a major role in data aggregation process, especially that aggregators are more attractive for attackers than normal nodes, where compromising few aggregators can significantly affect the final result of aggregation. In this paper, we present SAPC, a secure aggregation protocol for cluster-based WSN, which does not rely on trusted aggregator nodes and thus is immune to aggregators compromising. In addition to security performance, SAPC has a low transmission overhead