To protect against attacks where an adversary may compromise multiple servers over a long period of time, the use of secret share updates has been proposed. In this approach, the secret share of each server has to be periodically updated in collaboration with other servers in the system. Since the secret share's validity is limited in time, the adversary must compromise enough servers within a period of finite time to launch a successful attack.

The use of threshold cryptography to create a virtual CA makes two important assumptions regarding system initialization. First, it is assumed that Q servers can be initialized securely with their respective shares of the system secret. Second, it is assumed that each server can be configured securely with the public keys of all nodes which can potentially join the ad hoc network.

Both these assumptions basically boil down to the single assumption that the servers can be initially configured over a secure channel. This important assumption can sometimes act as a limitation in providing security in ad hoc networks.

One approach which has been proposed to reduce the dependency of the system on this assumption is localized self initialization. In this approach we still require that the first Q servers be initialized over a secure medium. However, once the first Q servers have been initialized, they can then collaborate to elect new servers.

This is achieved by having at least S servers use their secret share (k_x) to generate a partial secret share (ss_x). These partial secret shares are then combined to generate a new secret share which can be assigned to the node which is being initialized as a server. Let's do a short recap of how a virtual CA works in ad hoc networks.

As is true in any PKC system, each node in the ad hoc network has a private-key, public-key pair which it uses to secure communication with other nodes. To certify its keys, each node X, must have a valid certificate issued by the CA of the form K_iCA(X, K_wX, T_sign, T_expire).

This certificate basically says that the CA certifies (by signing the certificate using K_iCA) that the public key of node X is K_wX and this key is valid between times T_sign and T_expire. Such certificates which are signed using the system secret (K_iCA) are inherently trusted by all nodes in the network. It is these certificates which are then used to provide various security features in the network.

So, the aim of a virtual CA is to issue certificates signed using the system secret. The virtual CA is implemented as multiple physically separate nodes (servers) none of which knows the system secret (K_iCA) but each one of them knows a share of the system secret. When a node wants a certificate, it sends out a broadcast request. The servers then co-operate to supply the certificate thus providing security in the system.

Confidentiality and Integrity
Previously, we discussed how key establishment and authentication may be provided in multihop ad hoc networks. These two security services form the backbone of providing security in any network.

Once two nodes in a network have authenticated each other and securely established a security context (that is, securely established keys), encryption and integrity algorithms can be used to secure communication.

This part of system security is relatively simple. What is needed is the selection of algorithms and modes suitable for the environment in which the network is expected to operate.

Since the nodes in an ad hoc network environment usually have limited processing power and limited battery lifetimes, most ad hoc networks would prefer a stream cipher for encryption and an integrity algorithm which is not too computation intensive.

There are many stream ciphers to choose from as long as we keep in mind that there are some caveats while using stream ciphers in a wireless environment (as WEP demonstrated).