In multi-hop cellular networks, data packets have to be relayed hop by hop from a given mobile station to a base station and vice-versa. This means that the mobile stations must accept to forward information for the benefit of other stations. In this paper, we propose an incentive mechanism that is based on a charging/rewarding scheme and that makes collaboration rational for selfish nodes. We base our solution on symmetric cryptography to cope with the limited resources of the mobile stations. We provide a set of protocols and study their robustness with respect to various attacks. By leveraging on the relative stability of the routes, our solution leads to a very moderate overhead.

Mobile ad-hoc networks rely on the cooperation of nodes for routing and forwarding. For individual nodes there are however several advantages resulting from noncooperation, the most obvious being power saving. Nodes that act selfishly or even maliciously pose a threat to availability in mobile adhoc networks. Several approaches have been proposed to detect noncooperative nodes. In this paper, we investigate the e#ect of using rumors with respect to the detection time of misbehaved nodes as well as the robustness of the reputation system against wrong accusations. We propose a Bayesian approach for reputation representation, updates, and view integration. We also present a mechanism to detect and exclude potential lies. The simulation results indicate that by using this Bayesian approach, the reputation system is robust against slander while still benefitting from the speed-up in detection time provided by the use of rumors.

Reputation systems in mobile ad-hoc networks can be tricked by the spreading of false reputation ratings, be it false accusations or false praise. Simple solutions such as exclusively relying on one's own direct observations have drawbacks, as they do not make use of all the information available. We propose a fully distributed reputation system that can cope with false disseminated information. In our approach, everyone maintains a reputation rating and a trust rating about everyone else that they care about. From time to time first-hand reputation information is exchanged with others; using a modified Bayesian approach we designed and present in this paper, only second-hand reputation information that is not incompatible with the current reputation rating is accepted. Thus, reputation ratings are slightly modified by accepted information. Trust ratings are updated based on the compatibility of second-hand reputation information with prior reputation ratings. Data is entirely distributed: someone's reputation and trust is the collection of ratings maintained by others. We enable node redemption and prevent the sudden exploitation of good reputation built over time by introducing re-evaluation and reputation fading. We present the application of our generic reputation system to the context of neighborhood watch in mobile ad-hoc networks, specifically to the CONFIDANT [3] protocol for the detection and isolation of nodes exhibiting routing or forwarding misbehavior. We evaluate the performance by simulation.

Some misbehavior detection and reputation systems in mobile ad-hoc networks rely on the dissemination of information of observed behavior, which makes them vulnerable to false accusations. This vulnerability could be removed by forbidding the dissemination of information on observed behavior in the first place, but, as we show here, this has more drawbacks than a solution that allows dissemination and copes with false accusations. We propose a method for reducing the impact of false accusations. In our approach, nodes collect first-hand information about the behavior of other nodes by direct observation. In addition, nodes maintain a rating about every other node that they care about, in the form of a continuous variable per node. From time to time nodes exchange their first-hand information with others, but, using the Bayesian approach we designed and present in this paper, only second-hand information that is not incompatible with the current rating is accepted. Ratings are slightly modified by accepted information. The reputation of a given node is the collection of ratings maintained by others about this node. By means of simulation we evaluated the robustness of our approach against several types of adversaries that spread false information, and its e#ciency at detecting malicious nodes. The simulation results indicate that our system largely reduces the impact of false accusations, while still benefiting from the accelerated detection of malicious nodes provided by second-hand information. We also found that when information dissemination is not used, the time until malicious nodes are detected can be unacceptable. Key words: Mobile ad-hoc networks, misbehavior detection, reputation systems, Bayesian statistics Preprint submitted to Elsevier Science 1

Corporate IT as well as individuals show increasing interest in reliable outsourcing of storage infrastructure. Decentralized solutions with their resilience against partial outages are among the most attractive approaches. Irrespective of the form of the relationship, be it based on a contract or on the more flexible cooperative model, the problem of verifying whether someone promising to store one's data actually does so remains to be solved, especially in the presence of multiple replicas. In this paper, we introduce a lightweight mechanism that allows the data originator or a dedicated verification agent to build up trust in the replica holder by means of protocols that do not require prior trust or key establishment. We show how naive versions of the protocol do not prevent cheating, and then strengthen it by adding means that make it economically attractive to be honest. This provides a foundation for further work in providing trustworthy distributed storage.

Several misbehavior detection and reputation systems have been proposed for mobile ad-hoc networks, relying on direct network observation mechanisms, so-called watchdogs. While these approaches have so far only been evaluated in simulations and restricted to selfish packet dropping, we are interested in the capabilities of a watchdog detection component in a real network. In this paper we present our test-bed implementation of misbehavior detection. Following an evaluation of both the feasibility and detectability of attacks on routing and forwarding in the Dynamic Source Routing (DSR) protocol, we present the design of our test-bed. In order to add detection capabilities, we extend the concept of passive acknowledgment by mechanisms for partial dropping, packet modification, and fabrication detection. We combine DSR with Netfilter and APE to enable detection. We implement both attackers and detection and show their feasibility and limitations. 1

In multi-hop cellular networks, the existence of a communication link between the mobile station and the base station is not required: a mobile station that has no direct connection with a base station can use other mobile stations as relays. Compared with conventional (single-hop) cellular networks, this new generation can lead to a better usage of the available spectrum and to a reduction of infrastructure costs.

hybrid cellular ad-hoc networks, security, routing protocols While wireless networking seems to be the way of the future, no definite architecture for large scale deployment of such networks has emerged. This can be attributed to financial reasons (a specialized infrastructure is too expensive to build) as well as to the lack of solutions that could work with an existing infrastructure or, in an ad hoc manner, without one. A viable alternative seems to be hybrid wireless networks. Such networks use the existing cellular telephony infrastructure as basis and enhance it by building ad hoc networks of traffic relayers around each cell, improving the overall throughtput and reliability of the network. These relayers are users of the network that are willing to operate as such. In this setting, both efficiency and security are vital properties. We propose SCP as an integrated solution for secure routing and crediting in hybrid networks. We describe how a secure environment can be established efficiently by financially motivating users to avoid attacks. Finally, we show that SCP imposes minimal load both in communication and computation, so that even regular cellular phones can function as relayers, without demanding infrastructure upgrades. 1