Florian Tschorsch

Bitcoin and Beyond: A Technical Survey on Decentralized Digital Currencies

Besides attracting a billion dollar economy, Bitcoin revolutionized the field of digital currencies and influenced many adjacent areas. This also induced significant scientific interest. In this survey, we unroll and structure the manyfold results and research directions. We start by introducing the Bitcoin protocol and its building blocks. From there we continue to explore the design space by discussing existing contributions and results. In the process, we deduce the fundamental structures and insights at the core of the Bitcoin protocol and its applications. As we show and discuss, many key ideas are likewise applicable in various other fields, so that their impact reaches far beyond Bitcoin itself.

Tor is unfair — And what to do about it

Tor is one of the most popular network anonymization services. With increasing popularity, however, Tor is also faced with increasing load. Mechanisms for handling congestion and fairness in anonymization networks, where user privacy is of greatest significance, are not yet well understood. Thus current designs leave a lot to be desired: gross unfairness and largely suboptimal performance can be observed. In this paper, we focus on fairness aspects in the Tor network. We first show that interactions of multiple scheduling mechanisms in the current Tor design cause heavily unfair resource allocations to users. Subsequently, we develop a fairness model based on max-min fairness that takes the specifics of Tor into account. This leads us to a re-design of Tors scheduling. We implement the new design in conjunction with a congestion feedback mechanism named N23, which has recently been proposed to be used in Tor. Our scheduling approach overcomes the unfairness problems which are exhibited by todays Tor implementation, and by Tor with N23 as well. It achieves global max-min fairness and thus a fair resource allocation despite selfish end-users.

How (not) to build a transport layer for anonymity overlays

Internet anonymity systems, like for instance Tor, are in widespread use today. Technically they are realized as overlays, i. e., they add another instance of routing, forwarding, and transport functionality on top of the Internet protocol stack. This has important (and often subtle) implications, as overlay and underlay may interact. Far too often, existing designs neglect this. Consequently, they suffer from performance issues that are hard to track down and fix. The existing body of work in this area often takes a quite narrow view, tweaking the design in order to improve one specific aspect. The behavior of the interacting underlay and overlay transport layers is complex, though, and often causes unexpected-and unexplored-side effects. Therefore, we show that so far considered combinations of overlay and underlay protocols cannot deliver good throughput, latency, and fairness at the same time, and we establish guidelines for a future, better suited transport layer design.

Unleashing Tor, BitTorrent & Co.: How to relieve TCP deficiencies in overlays

In TCP-based overlay applications, the TCP connections of one peer typically share one physical Internet link. Using real-world experiments, we demonstrate that this can lead to undesirable interactions, causing significant throughput loss. We argue that such effects should be taken into account in the design of overlay networks, and identify readily deployable coun-termeasures. In a first step, we show that with existing operating system QoS functionality some relief is possible. Yet, this alone is not fully effective if peers communicate bidirectionally, due to piggybacked ACKs. We propose to separate bidirectionally used overlay links into two independent TCP connections, and demonstrate the effectiveness of this strategy.

An algorithm for privacy-preserving distributed user statistics

Abstract In this paper, we propose a privacy-preserving method to determine the number of distinct users who connected to one or more entry points of a distributed Internet service with multiple service operators. The problem is motivated by the anonymization network Tor, and the difficulties that arise when aiming to estimate the number of Tor users. We present a way to perform distributed user counting with accurate estimates and a high level of privacy protection, based on a probabilistic data structure. We start from a relatively naive approach, and analyze the level of privacy protection that it provides. Subsequently, we improve on this baseline mechanism, building upon the gained insights. In order to assess the privacy properties of the discussed techniques, we use a novel probabilistic analysis approach which compares an attacker’s a priori and a posteriori knowledge.

Towards a Concurrent and Distributed Route Selection for Payment Channel Networks

Payment channel networks use off-chain transactions to provide virtually arbitrary transaction rates. In this paper, we provide a new perspective on payment channels and consider them as a flow network. We propose an extended push-relabel algorithm to find payment flows in a payment channel network. Our algorithm enables a distributed and concurrent execution without violating capacity constraints. To this end, we introduce the concept of capacity locking. We prove that flows are valid and present first results.

ANOTEL: Cellular Networks with Location Privacy

While location management is a key component of cellular networks, it is also a major privacy issue: location management empowers the network operator to track users. In todays public and scientific discussion, the centralized storage of location data is mostly taken as a fact, and users are expected to trust the network operator. With ANOTEL we present a novel, clean-slate approach of location management in cellular networks that challenges this assumption. The design is able to route calls to users who move through cellular networks, without violating their location privacy.

CircuitStart: A Slow Start For Multi-Hop Anonymity Systems

In order to improve the performance of anonymity networks like Tor, custom transport protocols have been proposed to efficiently deal with the multi-hop nature of such overlay networks. In this work, we tackle the issue of quickly, but safely, ramping up the congestion window during the initial phase of a circuits lifetime. We propose a tailored startup mechanism called CircuitStart that transfers the idea of a traditional slow start to the multi-hop scenario by effectively compensating potential overshooting, improving performance compared to existing approaches.