Wireless communication networks have become crucial to modern society. By the end of 2004 there were approximately one billion mobile phones users. While in the early years of mobile phones the prevailing application was voice communication, now wireless data communication has become more important because of the technological advances of the internet. Computer and communication networks are usually centrally managed and supported by trusted network services. Examples are the base stations network in GSM and the router network of the internet. Recently, there has been a growing interest in networks, where a trusted infrastructure is not available. Such self-organized networks are called ad hoc networks and consist of autonomous nodes that collaborate in order to establish the desired network services. One important service that must be provided collaboratively is routing. Only nodes that are within each other’s transmission range can directly communicate over wireless links, while other nodes have to rely on their neighbors to relay messages to nodes which are further away. Hence, in such a multi-hop ad hoc network all nodes have to act both as hosts as well as routers.

Some possible civilian applications of ad hoc networks are:

• Spontaneous networking - a group of people come together and want to use collaborative applications, e.g. in meetings, conferences, classrooms, etc.
• Networking in locations where no other communication networks are available.
• Direct communication without involvement or charges from a network service provider or carrier.

There are many security issues in ad hoc networks that have to be solved. Two important issues focused on in our research are:

• Confidentiality on the wireless link. The wireless link is inherently susceptible to eavesdropping. Encryption of communication links between nodes is an important prerequisite for the widespread use of ad hoc networks. The main challenge here is the key establishment in the absence of a trusted third party.
• Robust against denial of service attacks carried out by selfish nodes. Each communication process uses the limited resources available to a node, namely its battery energy and the capacity of the wireless communication channel. Nodes that wish to preserve their resources could be reluctant to take part in the collaborative services although simultaneously avail of these services themselves. Nodes that act selfishly must be detected and excluded from the ad hoc network. Again, due to the absence of trusted authorities this monitoring approach has to be realized in a distributed way by all nodes of the ad hoc network.