## 1 Introduction  
Mobile _ad hoc_ networks (MANETs) are vital to the advancement of wireless networks. MANETs are characteristically composed of equal nodes that communicate over wireless links without any backbone. Some applications, such as strategic military communication, rescue missions in times of natural disasters and sensor networks, are just a few possible uses of MANETs [1]. The emergence of the MANET technology advocates self-organized wireless interconnection of communication devices that would either extend or operate in concert with the wired networking infrastructure or, possibly, advance to autonomous networks. In either case, the propagation of MANET-based applications depends on a multitude of factors [2]. MANETs generally inherit the conventional problems of wireless and mobile communications. These include issues related to bandwidth optimization, power control and transmission quality enhancement [1,3]. In addition, the multi-hop nature and absence of fixed infrastructure generate new problems, such as configuration advertising, discovery, maintenance, as well as _ad hoc_ addressing and self-routing. MANET topology is highly dynamic and random. Furthermore, the distribution of nodes and their ability to self-organize play an important role. Aside from their dynamic topology, due to their wireless transmission, MANETs are affected more by higher loss rates, higher delays and jitter than fixed networks. In addition, nodes in MANETs rely on batteries or other exhaustible power supplies for their energy. The most critical aspect of a MANET is that physical security is limited due to wireless transmission [1, 2, 3, 4, 5]. Security in MANETs mainly involves confidentiality and integrity of information, as well as legitimate use and availability of service [6]. Due to some characteristic features, MANETs are vulnerable to various security threats, such as eavesdropping, denial of service attacks (DoS), grey-hole attacks, black-hole attacks, _etc._[4]. In a grey-hole attack [7,8], the malicious node behaves correctly by sending valid route replies to nodes that initiate route requests. If the malicious node is chosen as the nearest node to the required destination, it periodically drops the data packet to launch a DoS [8, 9, 10] attack. In the case of a black hole attack [11], the malicious node waits for neighbouring nodes to send route request messages. It replies to the request by providing a route to the destination over itself. It assigns a high sequence number in the reply message, which enables it to be seen as the best route to the required destination. By taking over all of the route, the malicious node discards all of the packets passing through it, by launching a DoS attack [9,11]. The existence of well-known intrusion prevention mechanisms, such as cryptography or authentication, can reduce threats against MANETs, such as malicious data modification, which aims to reduce data integrity and confidentiality [12]. However, they cannot defend against malicious internal nodes that disrupt the routing of data. Furthermore, conventional cryptography and authentication mechanisms cannot effectively address the above-mentioned threats that arise due to the inherent broadcast wireless medium and decentralized architecture of MANETs. In general, the absence of a central authorization facility in an open and distributed communication environment is a major challenge, especially due to the need for cooperative network operation. Particularly in MANETs, any node may compromise the routing protocol functionality through a disruption of the route discovery process [2]. In order to ensure the availability of data in a MANET,all nodes in a MANET must act as routers for forwarding data packets from the source to the desired destinations [5]. It is therefore important for all nodes in a MANET to collaborate for the effective and efficient operations of the network.