Nymble: Anonymous IP-Address blocking

Anonymizing networks such as Tor allow users to access Internet services privately using a series of routers to hide the client's IP address from the server. Tor's success, however, has been limited by users employing this anonymity for abusive purposes, such as defacing Wikipedia. Website administrators rely on IP-address blocking for disabling access to misbehaving users, but this is not practical if the abuser routes through Tor. As a result, administrators block all Tor exit nodes, denying anonymous access to honest and dishonest users alike. To address this problem, we present a system in which (1) honest users remain anonymous and their requests unlinkable; (2) a server can complain about a particular anonymous user and gain the ability to blacklist the user for future connections; (3) this blacklisted user's accesses before the complaint remain anonymous; and (4) users are aware of their blacklist status before accessing a service. As a result of these properties, our system is agnostic to different servers' definitions of misbehavior.