I. Introduction
Network security has become a major concern in modern communication networks [1]. For wireless ad hoc networks (WANETs), protecting the secrecy of user messages is more challenging due to both the broadcast nature of wireless channel and the lack of central administration [2]. The traditional approach for guaranteeing the security in a wireless environment is to employ the cryptographic techniques at network layer [3]. However, the cryptographic-based method is not applicable in a resource-limited WANET since it incurs high computing complexity and thus the high energy consumption.