We consider two of the most important design issues for distributed sensor networks in the battlefield: security for communication in such hostile terrain; and energy efficiency because of battery?s limited capacity and the impracticality of recharging. Communication security is normally provided by encryption, i.e., data are encrypted before transmission and will be decrypted first on reception. We exploit the secure sensor network design space for energy efficiency by investigating different microprocessors coupled with various public key algorithms. We propose a power control mechanism for sensors to operate at an energy-efficient fashion using the newly developed dynamical voltage scaling (DVS) technique. In particular, we consider multiple voltage processors and insert additional information into the communication channel to guide the selection of proper voltages for data decryption/encryption and processing in order to reduce the total com-putational energy consumption. We experiment several encryption standards on a broad range of embedded processors and simulate the behavior of the sensor network to show that the sensor?s lifetime can be extended substantially.