I. Introduction
Wireless ad hoc networks provide an infrastructure-free communication over the shared wireless channel. Without the support of fixed infrastructure or access point, it opens a new wireless networking transmission paradigm by employing multi-hops for information transfer. The underlying self-organizing property provides an extremely flexible method for establishing communications in situations where geographical or terrestrial constraints demand totally distributed network, such as battlefields, emergency and disaster areas. While the great flexibility of wireless ad hoc networks also brings many research challenges. One big issue is the security problem. Developing a practical application of wireless ad hoc network depends largely on the provided security level. However, recent researches [1]–[3] indicate that the wireless ad hoc network is more vulnerable than the conventional wired and wireless networks due to its underlying characteristics of open medium, dynamic network topology, limited bandwidth, distributed cooperation and constrained energy resources. Existing security solutions for conventional network may not be adequate and cannot be applied directly to wireless ad hoc networks.