International Journal of Applied Information Systems (IJAIS) – ISSN : 2249-0868 Foundation of Computer Science FCS, New York, USA Volume 4– No.6, November 2012 – www.ijais.org 33 Distributed Certificate Management in Mobile Ad Hoc Networks Mohammad Masdari Computer Engineering Department, Islamic Azad University, Urmia Branch, Urmia, Iran, Javad Pashaei Barbin Computer Engineering Department, Islamic Azad University, Naghadeh Branch, Naghadeh, Iran, ABSTRACT PKI or public key infrastructure is used many security solutions that are designed for mobile ad hoc networks. These networks have special features that distinguish them from other wired and conventional networks and centralized Certificate Authorities cannot be used for certificate management in these kinds of networks. Thus many efforts have been made to adapt Certificate Authority’s (CA) tasks to the dynamic environments of MANETs and distribute the tasks of CA among MANET nodes. In this paper, we study various Certificate management solutions that are proposed in the literature and analyze their advantages and limitations. In addition, we emphasis on certificate revocation and validation issues and compare the overheads of these operations. Finally, we propose the characteristics of an ideal DCA system that can be used to verify the completeness of any DCA Scheme. Keywords Security, Distributed Certificate Authority, Threshold cryptography, Digital Signature. 1. INTRODUCTION Mobile ad hoc networks or MANETs are vulnerable to various passive and active security attacks that are launched by internal and external attackers. But because of special characteristics of MANETs, such as lack of any fixed infrastructure, mobility of nodes and limited bandwidth of wireless communication, establishing security in MANETs is a challenging issue. Numerous solutions have been designed and presented in the literature to increase the security level of these networks. Public key cryptography is used to provide privacy, integrity, authentication and other security services in Internet and other conventional networks. Certificates are on the main security data structures of PKI systems that assure the authenticity and integrity of public keys. Certificate authorities are trusted third parties that are used for issuing, revoking and managing of user certificates. But, MANETs lacks any infrastructure and are created dynamically by cooperation of mobile and wireless devices. Thus, for adapting PKI to mobile ad hoc networks, the tasks of certificate authority (CA) should be distributed on the user nodes or the functionality of CA should be emulated somehow. To solve all of these problems, the following certificate management solutions have been proposed in the literature:  Web of trust-based schemes.  Certificate Authority-based schemes. In the first case, every entity certifies the binding of identities and public-keys for other entities. In the second case, certificate are issued and managed by a certificate authority. Generally, the following two kinds of CA are used in the ad hoc networks:  Dependent CAs  Independent CAs Dependent CAs are used in the hybrid MANETs and certificate authority may depend on the centralized CA that resides on a fixed network. These, CAs act as a front-end to the main CA and support the MANET users’ requests by contacting the main CA. The independent CAs are useful in non-hybrid MANETs which are not connected to the fixed and wired networks. These distributed CAs are created dynamically and perform their operations distributedly by cooperation of ad hoc network nodes. In a Distributed Certificate Authority (DCA) private key is distributed among the shareholder nodes. When operations such as issuing or revoking certificates are required, threshold of shareholders participate to perform the requested service [4]. Like conventional CA, the public key of the DCA will be known by all network’s nodes and will be used to verify the signatures of certificates issued by the DCA. Many distributed certificate authorities schemes have been designed for MANET and they can be classified as the following items:  Partially distributed certificate authority  Fully distributed certificate authority In PDCAs the services of CA are distributed to a set of specialized server nodes using secret sharing. Each of these nodes can generate partial certificates and a client can create a valid certificate by combining enough number of these partial certificates. Therefore, PDCAs specially are useful in heterogonous MANETs which consist of some special nodes that have more processing and communication capabilities. However, in homogenous MANETs which all nodes are identical, the nodes of the distributed CA might be chosen randomly. In fully distributed certificate authorities or FDCA, all nodes became the DCA share holder and can generate partial certificates [15]. FDCA reduces the communication delay and improves the availability because almost all neighbors of a requesting node hold shares of the DCA’s private signature key. However, it is more vulnerable to malicious nodes and behaviors, because more nodes are part of the DCA. To overcome this problem, some schemes use trust management systems in combination with DCA systems and some other schemes use intrusion detection system to monitor the ad hoc network. Almost all FDCA and PDCA schemes use threshold cryptography which requires cooperation of k nodes from total n nodes. Thus k contact is need for each operation in the DCA which means that a client needs to contact at least k CA nodes and receive at least k replies. However it is possible that more than k, CA node receive the certificate request and respond to it, thus a client receives more responses than it needs. In [16], Luo et al, add the following items as properties of a distributed CA:  Liveness: The CA always processes a request in a finite amount of time.  Safety: An adversary is never able to forge a certificate.