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With the wide-spread of on-line Internet services and the increasing concern of users with their privacy, protocols that hide the identity of users are definitely emerging as a promising practical solution. In this talk we will focus on two important issues related to path-based anonymous protocols, which are usually designed to operate in a distributed and not necessarily trusted environment. We consider an attack, known as the predecessor attack, to break anonymity in such systems, and provide a statistical analysis of the effort required by an attacker to succeed. Our results provide tight bounds on its effectiveness and intuituvely, show that larger systems are harder to attack. The second issue deals with the vulnerability of anonymous systems to free-riders, peers that use the system while providing little or no service to others. We build on the idea of requiring currency to be exchanged in return for service, and propose a mechanism to allow anonymous digital cash payments to be made to those who provide service. This mechanism can be readily coupled with the class of peer-peer anonymous protocols that are based on Chaumian mixes while reserving their architectural simplicity. Finally, we formulate an abstract model of self-interested users in such a system and show that the payment based incentive mechanism can significantly improve the degree of anonymity by fostering greater cooperation among peers.
Joint work with: Don Towsley (UMass), Jonathan Shapiro (MSU) and Philippe Nain (INRIA)
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