Wireless ad hoc network possesses some unique characteristics that prove to be extremely valuable in mission-critical systems. However, the wide use of wireless ad hoc network in mission-critical systems is still limited by its poor security and constraints on energy consumption—the nodes in wireless ad hoc networks typically have limited battery supply. It is well known that the wireless medium provides greater opportunities for eavesdropping on the transmissions of data packets in the network. While encryption may be used to hide message contents in the packets, the observable traffic pattern might reveal important information about the system. Traffic padding may be used to hide the traffic pattern, which means to insert dummy traffic into the network and present to the intruder a different traffic pattern. The apparent traffic pattern, which is observed by intruder, is referred to as a cover mode that hides the real operation mode of the system. Since the dummy traffic incurs an overhead, it is important to minimize dummy traffic while achieving the desired security objective. This paper deals with the issue of designing energy-saving traffic padding schemes. Instead of simply reducing the amount of dummy traffic, we attempt to develop cover modes that optimize system parameters, i.e. system lifetime and energy consumption per unit of real traffic delivered by the network.