Anonymizing networks such as Tor allow users to access internet services privately by using a series of routers to hide the client’s IP address from the server. The success of such networks However has been limited by users employing this anonymity for abusive purposes such as defacing popular web sites. Web site administrators routinely rely on IP-address for blocking or disabling access to misbehaving users, but blocking IP addresses is not practical if the abuser routes through an anonymizing network. As a result, administrators block all known exit nodes of anonymizing networks, denying anonymous access misbehaving and behaving users alike. To address this problem, we present Nymble, a system in which servers can “blacklist” misbehaving users, thereby blocking users without compromising their anonymity. Our system is thus agnostic to different server definitions of misbehavior servers can blacklist users for whatever reason, and the privacy of blacklisted users is maintained.

Anonymizing networks such as Tor route traffic through independent nodes in separate administrative domains to hide a client’s IP address. Unfortunately, some users have misused such networks-under the cover of anonymity, users have repeatedly defaced popular Web sites. Since web site administrators cannot blacklist individual malicious users’ IP addresses, they blacklist the entire anonymizing network. Such measures eliminate malicious activity through anonymizing networks at the cost of denying anonymous access to behaving users. In other words, a few”bad apples” can spoil the fun for all. Subjective blacklisting is also better suited to servers such as Wikipedia, where misbehaviours such as questionable edits to a webpage, are hard to define in mathematical terms. In some systems, misbehavior can indeed be defined precisely. For instance, double spending of an “e-coin” is considered misbehavior in anonymous e-cash systems.

An anonymous P2P communication system is a peer-to-peer distributed application in which the nodes or participants are anonymous or pseudonymous. Anonymity of participants is usually achieved by special routing overlay networks that hide the physical location of each node from other participants. A comprehensive credential system called Nymble, which can be used to add a layer of accountability to any publicly known anonymizing network, Serves can blacklist misbehaving users while maintaining their privacy, and we show how these properties can be attained in a way that is practical, efficient and sensitive to the needs of both users and services.