In this paper, we propose an architecture for providing distributed Certificate Authority (CA) service in Mobile Ad Hoc Networks (MANET), based on threshold cryptography. We have two major contributions: 1) we make use of the cluster structure to provide CA service, and design a scheme for locating CA server nodes in MANET; 2) we provide a proactive secret share update protocol, which periodically updates CA secret shares with low system overhead. Compared with existing approaches, our CA architecture provides faster CA services to user nodes at reduced system overhead.

In PKI (Public Key Infrastructure), certificate revocation list (CRL) carries important information of the revoked certificates. Users need to check with this CRL frequently to make sure that the certificate in consideration is still valid and has not been revoked. In a wired network, this CRL is stored in a centralized Certificate Authority (CA) and can be accessed easily by users. However, in a mobile ad hoc network (MANET), which is characterized by its lack of infrastructure support, high node mobility, and the instability of communication links, we usually have to use multiple CAs. Users may not be able to connect to a particular CA, so distributing the latest version of CRL to all CAs becomes important. To solve this CRL distribution problem, a number of proposed solutions have been suggested. Most of these solutions are based on flooding. However, under what conditions flooding is a viable approach to distribute information in an ad hoc network has not been elaborated. In this paper, we take the first step to answer this question by simulation based on a realistic model. We identify some critical factors affecting the performance of flooding. And based on the simulation results, we provide useful insights on the minimum requirement for flooding to work smoothly in MANET.

When implementing public key security ser-easy for an adversary to collect data and attack. To solve this vices in mobile ad hoc networks (MANETs), multiple problem, the distributed CA [1] is proposed, and the funccertificate authority (CA) servers are usually adopted to tionality of a single CA is distributed to a set of nodes by increase the security of the system, with each CA node secret sharing and threshold cryptography: the private key of holding only one share of the private key. To prevent an the CA is distributed to multiple entities, with each of them adversary from collecting a large enough number of holding one share of the secret, and the CA service is obshares over a long period of time to compromise the sys-tained by accessing a number of shares. This system can tem, the shares will be periodically updated. However, it only be compromised if a large enough number of the secret is not trivial how this update procedure can be done effi-shares have been obtained by an adversary from multiple ciently in a MANET. In this paper, we devise an efficient locations. Obviously, this method reduces the risk of single distributed secret key share update scheme for MANETs point of failure. However, an adversary can still recover the based on the cluster architecture. In our scheme, the secret by accumulating enough number of shares one by one secret shares are updated first by a small group of server after a long period of time. To tackle this problem, the pronodes. With the assistance of the cluster head in each active share update scheme [1], which periodically refreshes cluster, the updated servers then refresh the shares in the shares, has been proposed. Thus, if the adversary cannot the remaining servers. We evaluate our scheme by simu-recover all the secrets within a limited period, all its efforts lation and show that our scheme can expedite the share are voided by the share update. update process.The proactive share update is critical for maintaining the security of the distributed CA approach. However, effi-I. INTRODUCTION ciently performing the proactive share update scheme in MANET is non-trivial. Traditional share update schemes Public Key Infrastructure (PKI) has been considered as require all CAs to participate in the process and collect parthe foundation of providing security services in a mobile ad tial shares from all other CAs in order to generate new hoc network (MANET). For example, many secure routing shares. The communication cost is high, especially when the protocols, such as SRP [10], and ARAN [9], assume that number of CAs is large. It is not practical in MANETs. PKI has been established. In traditional PKI, each entity hasTo solve this problem, [2] made the first attempt to proa public and private key pair. There is a trusted-by-all cenpose a scalable update scheme, called the sequential share tralized authority, called Certificate Authority (CA), responupdate scheme. Their scheme is designed for a fully distribsible for key management. The CA has a public and private uted CA thresh...