@conference {
	title = {Selfish response to Epidemic Propagation},
	author = {Theodorakopoulos, George., Le Boudec, Jean-Yves., Baras, John S.},
	year = {2011},
	month = {06/2011},
	abstract = {An epidemic spreading in a network calls for a decision on the part of the network members: They should decide whether to protect themselves or not. Their decision depends on the trade-off between their perceived risk of being infected and the cost of being protected. The network members can make decisions repeatedly, based on information that they receive about the changing infection level in the network. We study the equilibrium states reached by a network whose members increase (resp. decrease) their security deployment when learning that the network infection is higher (resp. lower). Our main result is that as the learning rate of the members increases, the equilibrium level of infection increases. We demonstrate this result both when members are strictly rational and when they are not. We characterize the domains of attraction of the equilibrium points.We validate our conclusions with simulations on human mobility traces.},
	address = {San Francisco, CA},
	booktitle = {2011 American Control Conference},
	pages = {pp. 4069-4074}
}
@conference {
	title = {Dynamic Network Security Deployment Under Partial Information},
	author = {Theodorakopoulos, George., Baras, John S., Le Boudec, Jean-Yves.},
	year = {2008},
	month = {09/2008},
	abstract = {A network user{\textquoteright}s decision to start and continue using security products is based on economic considerations. The cost of a security compromise (e.g., worm infection) is compared against the cost of deploying and maintaining a sufficient level of security. These costs are not necessarily the real ones, but rather the perceived costs, which depend on the amount of information available to a user at each time. Moreover, the costs (whether real or perceived) depend on the decisions of other users, too: The probability of a user getting infected depends on the security deployed by all the other users. In this paper, we combine an epidemic model for malware propagation in a network with a game theoretic model of the users{\textquoteright} decisions to deploy security or not. Users can dynamically change their decision in order to maximize their currently perceived utility. We study the equilibrium points~and their dependence on the speed of the learning process through which the users learn the state of the network. We find that the faster the learning process, the higher the total network cost.},
	address = {Monticello, IL},
	booktitle = {Forty-Sixth Annual Conference on Communication, Control and Computing},
	pages = {pp. 261-267}
}
@conference {
	title = {Intrusion Detection System Resiliency to Byzantine Attacks: The Case Study of Wormholes in OLSR},
	author = {Baras, John S., Radosavac, Svetlana., Theodorakopoulos, George., Sterne, Dan., Budulas, Peter., Gopaul, Richard.},
	year = {2007},
	month = {10/2007},
	abstract = {In this paper, we extend the work presented in [1] by quantifying the effects of in-band wormhole attacks on Intrusion Detection Systems. More specifically, we propose a mathematical framework for obtaining performance bounds of Byzantine attackers and the Intrusion Detection System (IDS) in terms of detection delay. We formulate the problem of distributed collaborative defense against coordinated attacks in MANET as a dynamic game problem. In our formulation, we have on the one hand a group of attackers that observe what is going on in the network and coordinate their attack in an adaptive manner. On the other side, we have a group of defending nodes (the IDS nodes) that collaboratively observe the network and coordinate their actions against the attackers. Using extensions of the game theoretic framework of [2] we provide a mathematical framework for efficient identification of the worst attacks and damages that the attackers can achieve, as well as the best response of the defenders. This approach leads to quantifying resiliency of the routing-attack IDS with respect to Byzantine attacks.},
	address = {Orlando, FL},
	booktitle = {2007 Military Communication Conference},
	pages = {pp. 1-7}
}
@conference {
	title = {Malicious Users in Unstructured Networks},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2007},
	month = {05/2007},
	abstract = {Unstructured networks (like ad-hoc or peer-to-peer networks) are networks without centralized control of their operation. Users make local decisions regarding whether to follow the network protocol or not. While providing scalability benefits, this degrades the performance, which is compounded by the potential presence of Malicious Users. In general, these users are trying to disrupt the operation of the network, and prevent the legitimate users from achieving their objectives. More specifically, they could try to break the connectivity of the network, or waste the resources of the legitimate users. In this work we use game theory to examine the effect of Malicious Users. All users are modeled as payoff-maximizing strategic agents. A simple model, fictitious play, is used for the legitimate user behavior, but no limits are imposed on the Malicious Users strategies. We look for the worst case equilibrium: the one that gives Malicious Users the highest payoff. We identify the importance of the network topology.},
	address = {Anchorage, Alaska},
	booktitle = {Proceedings of the 26th Annual IEEE Conference on Computer Communications (Infocom)},
	pages = {pp. 884-891}
}
@conference {
	title = {Coalition Formation in MANETS},
	author = {Jiang, Tao., Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {11/2006},
	abstract = {Wireless ad-hoc networks rely on the cooperation of participating nodes for almost all their functions. However, due to resource constraints, nodes are generally selfish and try to maximize their own benefit when participating in the network. Therefore, it is important to study mechanisms which can be used as incentives to form coalitions inside the network. In this paper, we study coalition formation based on game theory, especially cooperative game theory. First, the dynamics of coalition formation proceeds via pairwise bargaining. We show that the size of the maximum coalition is a decreasing function of the cost for establishing a link. After the coalition formation process reaches the steady state, we are interested in the stability of coalitions. We prove that coalitions are stable in terms of both pairwise stability and coalitional stability.},
	address = {Orlando, FL},
	booktitle = {25th Army Science Conference},
	pages = {pp.1-8}
}
@conference {
	title = {A Game for Ad Hoc Network Connectivity in the Presence of Malicious Users},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {11/2006},
	abstract = {Ad hoc network users are resource constrained: Before transmitting data, they have to take into account the energy expenditure involved. Even if a user is, in principle, willing to spend energy to improve network connectivity, his actual decision will be heavily influenced by the decisions of his neighboring users, since they act as relay nodes for him. Moreover, some of the neighbors may not be as benign as he is; in fact, they could be outright malicious. We are abstracting the tradeoff between spending energy and increasing connectivity by modeling the interaction of a user with his one-hop neighbors in a game theoretic fashion. Two types of users exist: Good users willingly trade energy for connectivity, but only if they expect their neighbors to do the same; Bad users try to destroy connectivity, but also lure the Good users to waste energy. Within our model for user behavior and sophistication, we explore outcomes that can arise in this graphical game.},
	address = {San Francisco, CA},
	booktitle = {2006 IEEE Globecom Technical Conference},
	pages = {pp. 1-5}
}
@conference {
	title = {A Testbed for Comparing Trust Computation Algorithms},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {11/2006},
	abstract = {Trust is the expectation of a person about another person{\textquoteright}s behavior. Trust is important for many security related decisions about, e.g., granting or revoking privileges, controlling access to sensitive resources and information, or evaluating intelligence gathered from multiple sources. More often than not, the issue is complicated even further because the person making the decision has no direct trust relationship with every single subject whose trustworthiness needs to be evaluated. So, the decision maker needs to rely on recommendations by others, and then somehow aggregate the trust related information that is collected. In this work we provide an algebraic framework in which we can describe multiple ways that trust related information can be aggregated to form a single value. We show the similarities and differences that the various so called trust computation algorithms have, and associate these with the algebraic properties of the frame- work that we consider.},
	address = {Orlando, FL},
	booktitle = {25th Army Science Conference},
	pages = {pp. 1-8}
}
@conference {
	title = {Enhancing Benign User Cooperation in the Presence of Malicious Adversaries in Ad Hoc Networks},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {08/2006},
	abstract = {Decentralized and unstructured networks are becoming more prevalent today (e.g. ad hoc networks). Like every network, they depend on the cooperation of their users to survive. However, each user does not necessarily know who the others are, or what their intentions are. Since there is no centralized infrastructure, the users can only base their decision on what they observe themselves. Ideally, they would like to cooperate only with users that have common interests with them. In this paper, we use a game theoretic model for the above situation. We assume there are only two kinds of users, Good (benign) and Bad (malicious). Good users receive a high game theoretic payoff when they cooperate with other Good users, but a low payoff when they cooperate with Bad users. We propose behavior rules (strategies) to achieve equilibria that enable as many Good users as possible to cooperate with each other, and at the same time minimize the number of Good-Bad cooperations.},
	address = {Baltimore, MD},
	booktitle = {2006 IEEE Communications Society/CreateNet International Conference on Security and Privacy in Communication Networks}
}
@conference {
	title = {Linear Iterations on Ordered Semirings for Trust Metric Computation and Attack Resiliency Evaluation},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {07/2006},
	abstract = {Within the realm of network security, we interpret the concept of trust as a relation among entities that participate in various protocols. Trust relations are based on evidence created by the previous interactions of entities within a protocol. In this work, we are focusing on the evaluation of trust evidence in Ad Hoc Networks. Because of the dynamic nature of Ad Hoc Networks. Because of the dynamic nature of Ad Hoc Networks, trust evidence may be uncertain and incomplete. Also, no pre-established infrastructure can be assumed. The evaluation process is modelled as a path problem on a directed graph, where nodes represent entities, and edges represent trust relations. We develop a novel formation of trust computation as linear interactions on ordered semirings. Using the theory of semirings, we analyze several key problems on the performance of trust algorithms. We also analyze the resilience to attacks of the resulting schemes.},
	address = {Kyoto, Japan},
	booktitle = {Proceedings of the 17th International Symposium on Mathematical Networks and Systems},
	pages = {pp. 509-514}
}
@conference {
	title = {On Trust Models and Trust Evaluation Metrics for Ad Hoc Networks},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {02/2006},
	abstract = {Within the realm of network security, we interpret the concept of trust as a relation among entities that participate in various protocols. Trust relations are based on evidence created by the previous interactions of entities within a protocol. In this work, we are focusing on the evaluation of trust evidence in ad hoc networks. Because of the dynamic nature of ad hoc networks, trust evidence may be uncertain and incomplete. Also, no preestablished infrastructure can be assumed. The evaluation process is modeled as a path problem on a directed graph, where nodes represent entities, and edges represent trust relations. We give intuitive requirements and discuss design issues for any trust evaluation algorithm. Using the theory of semirings, we show how two nodes can establish an indirect trust relation without previous direct interaction. We show that our semiring framework is flexible enough to express other trust models, most notably PGP{\textquoteright}s Web of Trust. Our scheme is shown to be robust in the presence of attackers.},
	booktitle = {IEEE Journal on Selected Areas in Communications},
	pages = {pp. 318-328}
}
@conference {
	title = {Coalition Formation in MANET and Other Networks},
	author = {Jiang, Tao., Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {01/2006},
	abstract = {Wireless ad-hoc networks rely on the cooperation of participating nodes for almost all their functions. However, due to resource constraints, nodes are generally selfish and try to maximize their own benefit when participating in the network. Therefore, it is important to study mechanisms which can be used as incentives to form coalitions inside the network. In this paper, we study coalition formation based on game theory, especially cooperative game theory. First, the dynamics of coalition formation proceeds via pairwise bargaining. We show that the size of the maximum coalition is a decreasing function of the cost for establishing a link. After the coalition formation process reaches the steady state, we are interested in the stability of coalitions. We prove that coalitions are stable in terms of both pairwise stability and coalitional stability.},
	address = {Orlando, FL},
	booktitle = {25th Army Science Conference}
}
@conference {
	title = {Trust Evaluation in Ad-Hoc Networks},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2004},
	month = {10/2004},
	abstract = {An important concept in network security is trust, interpreted as a relation among entities that participate in various protocols. Trust relations are based on evidence related to the previous interactions of entities within a protocol. In this work, we are focusing on the evaluation process of trust evidence in Ad Hoc Networks. Because of the dynamic nature of Ad Hoc Networks, trust evidence may be uncertain and incomplete. Also, no pre-established infrastructure can be assumed. The process is formulated as a path problem on a directed graph, where nodes represent entities, and edges represent trust relations. Using the theory of semirings, we show how two nodes can establish an indirect trust relation without previous direct interaction. The results are robust in the presence of attackers. We give intuitive requirements for any trust evaluation algorithm. The performance of the scheme is evaluated on three topologies.},
	address = {Philadelphia, PA},
	booktitle = {2004 ACM workshop on Wireless security},
	pages = {pp. 1-10}
}
@article {
	title = {Selfish response to Epidemic Propagation},
	author = {Theodorakopoulos, George., Le Boudec, Jean-Yves., Baras, John S.},
	year = {2013},
	month = {02/2013},
	abstract = {An epidemic that spreads in a network calls for a decision on the part of the network users. They have to decide whether to protect themselves or not. Their decision depends on the tradeoff between the perceived infection and the protection cost. Aiming to help users reach an informed decision, security advisories provide periodic information about the infection level in the network. We study the best-response dynamic in a network whose users repeatedly activate or de-activate security, depending on what they learn about the infection level. Our main result is the counterintuitive fact that the equilibrium level of infection increases as the users{\textquoteright} learning rate increases. The same is true when the users follow smooth best-response dynamics or any other continuous response function that implies a higher probability of protection when learning a higher level of infection. In both cases, we characterize the stability and the domains of attraction of the equilibrium points. Our finding also holds when the epidemic propagation is simulated on human contact traces, both when all users are of the same best-response behavior type and when they are of two distinct behavior types.},
	pages = {pp. 363 - 376},
	volume = {Volume 58.},
	number = {Issue 2},
	journal = {IEEE Transactions on Automatic Control.}
}
@article {
	title = {Game Theoretic Modeling of Malicious Users in Collaborative Networks},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2008},
	month = {09/2008},
	abstract = {If a network is to operate successfully, its users need to collaborate. Collaboration takes the form of following a network protocol and involves some resource expenditure on the part of the user. Therefore, users cannot automatically be expected to follow the protocol if they are not forced to. The situation is exacerbated by the presence of malicious users whose objective is to damage the network and increase the cost incurred by the legitimate users. The legitimate users are, at least initially, unaware of the type (legitimate or malicious) of the other users. Our contribution is a model for the strategic interaction of legitimate and malicious users as described above. The model is based on repeated graphical games with incomplete information. We describe and analyze two specific instantiations, aiming to demonstrate the model{\textquoteright}s expressive power and tractability. The main benefit we see from using game theory for this essential security problem is the ability to bound the damage caused by the malicious users.},
	pages = {pp.1317-1327},
	volume = {Vol. 26},
	number = {Issue. 7},
	journal = {IEEE Journal on Selected Areas in Communications}
}
@article {
	title = {On Trust Models and Trust Evaluation Metrics for Ad-Hoc Networks},
	author = {Theodorakopoulos, George., Baras, John S.},
	year = {2006},
	month = {02/2006},
	abstract = {Within the realm of network security, we interpret the concept of trust as a relation among entities that participate in various protocols. Trust relations are based on evidence created by the previous interactions of entities within a protocol. In this work, we are focusing on the evaluation of trust evidence in ad hoc networks. Because of the dynamic nature of ad hoc networks, trust evidence may be uncertain and incomplete. Also, no preestablished infrastructure can be assumed. The evaluation process is modeled as a path problem on a directed graph, where nodes represent entities, and edges represent trust relations. We give intuitive requirements and discuss design issues for any trust evaluation algorithm. Using the theory of semirings, we show how two nodes can establish an indirect trust relation without previous direct interaction. We show that our semiring framework is flexible enough to express other trust models, most notably PGP{\textquoteright}s Web of Trust. Our scheme is shown to be robust in the presence of attackers.},
	pages = {pp. 318-328},
	volume = {Vol. 24},
	number = {Number 2},
	journal = {Journal of Selected Areas in Communications, Security in Wireless Ad-Hoc Networks [2007~IEEE Communications Society Leonard G. Abraham Prize]}
}
