Nymbler: Privacy-enhanced Protection from Abuses of Anonymity
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Anonymous communications networks help to solve the real and important problem of enabling users to communicate privately over the Internet. However, by doing so, they also introduce an entirely new problem: How can service providers on the Internet---such as websites, IRC networks and mail servers---allow anonymous access while protecting themselves against abuse by misbehaving anonymous users? Recent research efforts have focused on using anonymous blacklisting systems (also known as anonymous revocation systems) to solve this problem. As opposed to revocable anonymity systems, which enable some trusted third party to deanonymize users, anonymous blacklisting systems provide a way for users to authenticate anonymously with a service provider, while enabling the service provider to revoke access from individual misbehaving anonymous users without revealing their identities. The literature contains several anonymous blacklisting systems, many of which are impractical for real-world deployment. In 2006, however, Tsang et al. proposed Nymble, which solves the anonymous blacklisting problem very efficiently using trusted third parties. Nymble has inspired a number of subsequent anonymous blacklisting systems. Some of these use fundamentally different approaches to accomplish what Nymble does without using third parties at all; so far, these proposals have all suffered from serious performance and scalability problems. Other systems build on the Nymble framework to reduce Nymble's trust assumptions while maintaining its highly efficient design. The primary contribution of this thesis is a new anonymous blacklisting system built on the Nymble framework---a nimbler version of Nymble---called Nymbler. We propose several enhancements to the Nymble framework that facilitate the construction of a scheme that minimizes trust in third parties. We then propose a new set of security and privacy properties that anonymous blacklisting systems should possess to protect: 1) users' privacy against malicious service providers and third parties (including other malicious users), and 2) service providers against abuse by malicious users. We also propose a set of performance requirements that anonymous blacklisting systems should meet to maximize their potential for real-world adoption, and formally define some optional features in the anonymous blacklisting systems literature. We then present Nymbler, which improves on existing Nymble-like systems by reducing the level of trust placed in third parties, while simultaneously providing stronger privacy guarantees and some new functionality. It avoids dependence on trusted hardware and unreasonable assumptions about non-collusion between trusted third parties. We have implemented all key components of Nymbler, and our measurements indicate that the system is highly practical. Our system solves several open problems in the anonymous blacklisting systems literature, and makes use of some new cryptographic constructions that are likely to be of independent theoretical interest.
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