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 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 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.
Existing users’ credentials must be updated, making it impractical. Verifier-local revocation (VLR) fixes this shortcoming by requiring the server (“verifier”) to perform only local updates during revocation. Unfortunately, VLR requires heavy computation at the server that is linear in the size of the blacklist.
We present a secure system called Nymble, which provides all the following properties: anonymous authentication, backward unlink ability, subjective blacklisting, fast authentication speeds, rate-limited anonymous connections, revocation audit ability (where users can verify whether they have been blacklisted), and also addresses the Sybil attack to make its deployment practical In Nymble, users acquire an ordered collection of nymbles, a special type of pseudonym, to connect to websites. Without additional information, these nymbles are computationally hard to link, and hence using the stream of nymbles simulates anonymous access to services.
Websites, however, can blacklist users by obtaining a seed for a particular nymble, allowing them to link future nymbles from the same user — those used before the complaint remains unlink able. Servers can therefore blacklist anonymous users without knowledge of their IP addresses while allowing behaving users to connect anonymously. Our system ensures that users are aware of their blacklist status before they present a nymble, and disconnect immediately if they are blacklisted. Although our work applies to anonymizing networks in general, we consider Tor for purposes of exposition. In fact, any number of anonymizing networks can rely on the same Nymble system, blacklisting anonymous users regardless of their anonymizing network(s) of choice.
• Blacklisting anonymous users.
We provide a means by which servers can blacklist users of an anonymizing network while maintaining their privacy.
• Practical performance.
Our protocol makes use of inexpensive symmetric cryptographic operations to significantly outperform the alternatives.
• Open-source implementation.
With the goal of contributing a workable system, we have built an opensource implementation of Nymble, which is publicly available. We provide performance statistics to show that our system is indeed practical.
- Nymble Manager
- Pseudonym Manager
- Blacklisting a user
- Nymble-authenticated connection