In this paper, we explore traffic analysis attacks on Tor that are conducted solely with middle relays rather than with relays from the entry or exit positions. We create a methodology to apply novel Tor circuit and website fingerprinting from middle relays to detect onion service usage; that is, we are able to identify websites with hidden network addresses by their traffic patterns. We also carry out the first privacypreserving popularity measurement of a single social networking website hosted as an onion service by deploying our novel circuit and website fingerprinting techniques in the wild. Our results show: (i) that the middle position enables wide-scale monitoring and measurement not possible from a comparable resource deployment in other relay positions, (ii) that traffic fingerprinting techniques are as effective from the middle relay position as prior works show from a guard relay, and (iii) that an adversary can use our fingerprinting methodology to discover the popularity of onion services, or as a filter to target specific nodes in the network, such as particular guard relays.

Abstract. Tor is the most popular low-latency anonymity network for enhancing ordinary users' online privacy and resisting censorship. While it has grown in popularity, Tor has a variety of performance problems that result in poor quality of service, a strong disincentive to use the system, and weaker anonymity properties for all users. We observe that one reason why Tor is slow is due to lowbandwidth volunteer-operated routers. When clients use a low-bandwidth router, their throughput is limited by the capacity of the slowest node. With the introduction of bridges-unadvertised Tor routers that provide Tor access to users within censored regimes like China-low-bandwidth Tor routers are becoming more common and essential to Tor's ability to resist censorship. In this paper, we present Conflux, a dynamic traffic-splitting approach that assigns traffic to an overlay path based on its measured latency. Because it enhances the load-balancing properties of the network, Conflux considerably increases performance for clients using low-bandwidth bridges. Moreover, Conflux significantly improves the experience of users who watch streaming videos online. Through live measurements and a whole-network evaluation conducted on a scalable network emulator, we show that our approach offers an improvement of approximately 30% in expected download time for web browsers who use Tor bridges and for streaming application users. We also show that Conflux introduces only slight tradeoffs between users' anonymity and performance.

An increasing number of countries implement Internet censorship at different scales and for a variety of reasons. In particular, the link between the censored client and entry point to the uncensored network is a frequent target of censorship due to the ease with which a nation-state censor can control it. A number of censorship resistance systems have been developed thus far to help circumvent blocking on this link, which we refer to as link circumvention systems (LCs). The variety and profusion of attack vectors available to a censor has led to an arms race, leading to a dramatic speed of evolution of LCs. Despite their inherent complexity and the breadth of work in this area, there is no systematic way to evaluate link circumvention systems and compare them against each other. In this paper, we (i) sketch an attack model to comprehensively explore a censor's capabilities, (ii) present an abstract model of a LC, a system that helps a censored client communicate with a server over the Internet while resisting censorship, (iii) describe an evaluation stack that underscores a layered approach to evaluate LCs, and (iv) systemize and evaluate existing censorship resistance systems that provide link circumvention. We highlight open challenges in the evaluation and development of LCs and discuss possible mitigations.

Tor is the most popular low-latency anonymity overlay network for the Internet, protecting the privacy of hundreds of thousands of people every day. To ensure a high level of security against certain attacks, Tor currently utilizes special nodes called entry guards as each client's long-term entry point into the anonymity network. While the use of entry guards provides clear and well-studied security benefits, it is unclear how well the current entry guard design achieves its security goals in practice.We design and implement Changing of the Guards (COGS), a simulation-based research framework to study Tor's entry guard design. Using COGS, we empirically demonstrate that natural, shortterm entry guard churn and explicit time-based entry guard rotation contribute to clients using more entry guards than they should, and thus increase the likelihood of profiling attacks. This churn significantly degrades Tor clients' anonymity. To understand the security and performance implications of current and alternative entry guard selection algorithms, we simulate tens of thousands of Tor clients using COGS based on Tor's entry guard selection and rotation algorithms, with real entry guard data collected over the course of eight months from the live Tor network.

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