## Computer Science Technical Reports

Technical Report

12-7-2008

TR2008-637

#### Abstract

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 websites. Website administrators routinely rely on IP-address blocking for 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 \emph{all} known exit nodes of anonymizing networks, denying anonymous access to misbehaving and behaving users alike. To address this problem, we present Nymble, a system in which servers can blacklist'' misbehaving users, thereby \emph{blocking users without compromising their anonymity}. Our system is thus agnostic to different servers' definitions of misbehavior --- servers can blacklist users for whatever reason, and the privacy of blacklisted users is maintained.