Blocking in Tor with Trusted Computing ( Short Paper : Work in Progress ) ∗

How does one block an anonymous user hiding behind an anonymous routing network? In this paper, we outline a security protocol that uses resourceconstrained trusted hardware to facilitate anonymous IP-address blocking in anonymizing networks such as Tor. Tor allows users to access Internet services privately by using a series of Tor routers to obfuscate the route from the client to the server, thereby hiding the client’s IP address from the server. The success of Tor, however, has been limited because of malicious users who misuse the network. For example, anonymous users can deface websites or create malicious entries on websites such as Wikipedia. Administrators of these websites routinely rely on IP-address blocking for disabling misbehaving users’ accesses. The IP-address anonymity provided by Tor, however, makes it difficult for administrators to deny access to such offenders. As a result, administrators resort to blocking all Tor exit nodes, effectively denying anonymous access for all Tor’s users. Our solution makes use of trusted hardware and allows services like Tor to provide anonymous blocking of IP addresses while requiring only a modest amount of storage at the trusted node. ∗This research was supported in part by the NSF, under grant CNS-0524695, and the Bureau of Justice Assistance, under grant 2005-DD-BX-1091. The views and conclusions do not necessarily reflect the views of the sponsors. 1Wikipedia. http://www.wikipedia.com