Information aggregation is an important operation in wireless sensor networks executed for the purpose of monitoring and reporting of the environmental data. Due to the performance constraints of sensor nodes the in-network form of the aggregation is especially attractive since it allows to save expensive resources during the frequent network queries. Easy accessibility of networks and nodes and almost no physical protection against corruptions arise high challenges on the security of the aggregation process. Especially, protection against attacks aiming to falsify the aggregated result is considered to be of prime importance. In this paper we propose a novel security model for the aggregation process based on the well-established cryptographic techniques, focusing on the scenario with the single aggregator node. In order to show soundness and feasibility of our definitions we describe a generic practical approach that achieves security against node corruptions during the aggregation p...