A Distributed Trust Establishment Scheme for Mobile Ad Hoc Networks Jaydip Sen 1 , Piyali Roy Chowdhury 2 , Indranil Sengupta 1 1 Department of Computer Science and Engineering Indian Institute of Technology, Kharagpur-721302, INDIA 2 Department of Computer Science and Engineering Future Institute of Engineering and Management, Kolkata-700150, INDIA 1 sen_jaydip@yahoo.com, 2 roychowdhury.piyali@gmail.com, 1 isg@iitkgp.ac.in Abstract Wireless ad hoc networks have generated much interest, both in research literature and the telecommunication industry. The attractiveness of these networks lies in the fact that unlike other wireless networks, ad hoc networks are self-organized: the hosts constituting the networks can communicate with each other without reliance on centralized or specialized entities such as base stations. As these networks find more application, the need for adequate security mechanism is increasingly becoming important. Trust establishment and management are essential for any security framework of these networks. In this paper, we present a trust establishment scheme for ad hoc networks based on distributed trust model. A trust initiator is introduced only in the system-bootstrapping phase to initiate the protocol. A fully self-organized trust establishment approach is then adopted to handle the dynamic topology of the network and the membership changes of the nodes, while ensuring trust establishment among the nodes with shorter trust chains and very high probability. The simulation results show that our scheme is highly robust and scalable in the dynamic environment of ad hoc networks. 1. Introduction An ad hoc network is a group of mobile nodes without a centralized administration or a fixed network infrastructure. Due to their distributed nature, ad hoc networks are vulnerable to various types of attacks [1][2][3]. One strategy to improve security of ad hoc networks is to develop mechanisms that allow a node to evaluate trustworthiness of other nodes. Such mechanisms not only help in detection of malicious node(s), but also improve network performances because honest nodes can avoid working with untrustworthy nodes. In traditional wired networks, most trust evidences are generated via potentially lengthy assurance processes, distributed off- line, assumed to be valid for a long-term and certain at the time when trust relations derived from it are exercised [4]. In contrast, few of these characteristics of trust relations and trust evidences are prevalent in mobile ad hoc networks. Lack of fixed networking infrastructure, high mobility of the nodes, limited range and lack of reliability of the wireless links are some of the characteristics of ad hoc networks that make design of a trust establishment scheme a very difficult and challenging task. In particular, trust relations may have to be established using only on- line evidence and may be short-term and largely peer-to- peer. Since solutions developed for the fixed wireline networks are not suitable in such a scenario, new security solutions are eagerly in demand. In this paper, we present a distributed trust establishment scheme for ad hoc networks where a trust initiator is introduced in the bootstrapping phase of the system to initiate the process of trust establishment. With the help of the trust initiator, sufficient trust relationships are established so that any pair of nodes in the network can authenticate each other with a very high probability via a trust chain. The scheme provides high reliability in a dynamic network environment by ensuring that there exist at least two independent trust chains between any pair of nodes. The average length of the trust chains is made as short as possible to make the process efficient and avoid trust dilution. We present an analysis based on random trust graph approach to show the feasibility of the proposed scheme. The simulation results clearly demonstrate the effectiveness of the scheme. The salient features of the scheme can be summarized as follows: (1) The average length of the shortest indirect trust-paths between any pair of nodes is very small making the scheme more secure as there are fewer intermediate nodes in a trust path. (2) The average number of shortest indirect trust-paths between any pair of nodes is guaranteed to be at least two, each with a very high probability ensuring robustness and reliability of the scheme. (3) The scheme is highly adaptable to dynamic nature of ad hoc networks that have high mobility and fast changing topology. The rest of the paper is organized as follows: Section 2 reviews some related works on trust management in ad hoc networks. Section 3 presents the details of the proposed scheme, Section 4 gives the mathematical analysis of the scheme. Section 5 describes the simulation and analysis of the observed results, and finally Section 6 concludes the paper while highlighting some possible future directions of work.