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In many applications of sensor networks, readings from sensor nodes are aggregated at intermediate nodes to reduce the communication cost. The messages that are relayed in the data aggregation hierarchy may need confidentiality. We present a secure data aggregation protocol for sensor networks that uses encryption for confidentiality, but without requiring decryption at intermediate nodes. A salient feature of the protocol is the use of two-hop pairwise keys to provide integrity while minimizing the communication required between the base station and sensor nodes. We analyze the performance of our protocol and compare its efficiency with a protocol proposed by Hu and Evans.