Related Work

There has been much research focused on the mobile ad hoc network security. In [8], Zhou and Hass proposed a secure key management scheme. They used (n, k) threshold cryptography to distribute trust among a set of servers. They focused on the security of the shared secret in the presence of possible compromises of secret share holders. The system can tolerate k-1 compromised servers. However, they did not provide any specific explanation for how a node can make contact to sufficient servers, especially when the servers are spread across a large area. The authors also proposed to employ proactive schemes to achieve share refreshing to counter mobile adversaries. Yet, their solution assumed the group of servers with rich connectivity. It is not suitable for ad hoc environments. The authors also did not address the issue of how to distribute the update shares to the server nodes efficiently and securely.

In [9], Kong et al. also used threshold secret sharing mechanism to distribute the

functions of CA to some nodes. In order to ease the difficulty of contacting server nodes, they employed localized certification schemes. In other words, each entity holds a secret share, and multiple entities in a local neighborhood then jointly provide complete services. This method also can enhance the service efficiency for users. The authors noted that k is the balance point between service availability and intrusion tolerance. However, in their scheme the threshold value k is difficult to set. It is known that if k is too small, the probability of a global secret key being compromised is quite high. On the other hand, if k is big, although we can resist more compromises, it is relatively harder to find k one-hop legitimate neighboring nodes.

Another problem is that they also did not address the issue of how to distribute the update shares to the server nodes efficiently and securely. [10] is an extension of [9] because the authors proposed the parallel share updates to prevent from emulating a coalition of k nodes to fake share updates. Yet, this method requires a much higher communication cost due to the fact that each update polynomial function has to be generated by k nodes collaboratively.

After the polynomial function is generated, each node that wants to do the update procedure would be required to ask k nodes again to decrypt the polynomial function, then to complete the update. The authors also implemented a localized certification service to enhance service availability for mobile nodes and robustness against DoS attacks. However, this localized certification service operates under the assumption that each node has at least k legitimate neighbors; it surely has some difficulties to ensure that in a mobile ad hoc network

environment.

In [11], Bechler et al. proposed and evaluated a clustered architecture for securing communication in mobile ad hoc networks. They divided the network into clusters and used threshold cryptography to implement a decentralized CA. The authors further separated the cluster internal traffic from the network-wide traffic. For cluster internal traffic, they used the symmetric encryption. For network-wide traffic, they used public key cryptography. There are,

however, two major problems with their proposed architecture: (1) with its log-on procedure and (2) with its sharing update. First, they did not specify how to find the warrant nodes, and also the number of warrant nodes is indeterminate. Second, they used proactive secret sharing without any modification. Therefore, the communication overhead is too high for the wireless channel.

In [12], Zhu et al. proposed a novel key management scheme based on the hierarchical structure and secret sharing to distribute cryptography keys and to provide certification services, called the Autonomous Key Management (AKM). AKM is a logical tree, in which all the left nodes represent real wireless nodes, while all the branch nodes only exist logically.

AKM can achieve flexibility and adaptivity by issuing certificates with different levels of assurance and can handle the mobile ad hoc network (MANET) with a large number of nodes.

They further proposed two algorithms, which are based on threshold cryptography and Verifiable Secret Sharing (VSS). These algorithms can resist active attacks targeting

certification services. The disadvantage of AKM is that if we want to change the configuration of (n, k) to (n’, k’), it would require a significant cost. Under their “join operation,” when one real node wants to join a region, the system would choose a group of k nodes randomly. The authors assumed that each node in that group should know the identity of one another.

However, their assumption is not sound, since each node is randomly chosen.

In [13], Wu et al. also adopted the threshold cryptography to distribute the private key share to shareholders. The major difference in this presented model is that the shareholders form a special group, called the server group. In the pervious approach the shareholders are all independent; users must communicate with each server node individually. In contrast, here a user only needs to communicate with one member of the server group, then that server node will send the information to other server nodes automatically. The advantage of this method is that it is easier for a node to request service from a well maintained group rather than from multiple “independent” service providers, which may spread across a large area. Furthermore,

the server group does not have to include all shareholders; it takes the soft state maintenance to ensure a number of shareholders. In sum, the benefits include communication-efficiency, bandwidth-saving, and easy management. However, the size of a server group is the

determinant of the entire network performance. That is, if the server group is small, it is then relatively easier to respond and manage. Yet, this kind of small server groups may not have the ability to serve a large network. On the other hand, if the group is big, the response rate would, as expected, be slower and thus would have an impact on the entire network

performance. Therefore, how to decide on the size of a server group would be key in determining the quality of network performances.