To reduce energy consumption, aggregation takes place in a wireless sensor network. All measured data is collected and preprocessed multiple times on its way towards a data sink, e.g., a base station. However, aggregation implies new challenges to security: as the sink finally receives aggregated data, it is difficult to verify not only the aggregate's correctness, but also the origin of the data the aggregate was computed from. In the presence of an attacker in the network, data transmissions and aggregation could have maliciously been modified. Yet, it turns out that in-network aggregation and data authenticity are contradictory communication properties. This research examines the possibility of finding a trade-off between security (authenticity) and energy-savings (aggregation). If the user is willing to accept data's authenticity with $p\leq{}100\%$ probability, he can still save large amounts of energy compared to authentic communication without aggregation.