Private and Censorship-Resistant Communication over Public Networks Michael John Rogers Thesis submitted for the degree of Doctor of Philosophy of UCL Department of Computer Science University College London University of London 2010 1 I, Michael John Rogers, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 2 Abstract Society’s increasing reliance on digital communication networks is creating unprecedented opportunities for whole- sale surveillance and censorship. This thesis investigates the use of public networks such as the Internet to build robust, private communication systems that can resist monitoring and attacks by powerful adversaries such as national governments. We sketch the design of a censorship-resistant communication system based on peer-to-peer Internet overlays in which the participants only communicate directly with people they know and trust. This ‘friend-to-friend’ approach protects the participants’ privacy, but it also presents two significant challenges. The first is that, as with any peer-to-peer overlay, the users of the system must collectively provide the resources necessary for its operation; some users might prefer to use the system without contributing resources equal to those they consume, and if many users do so, the system may not be able to survive. To address this challenge we present a new game theoretic model of the problem of encouraging cooperation between selfish actors under conditions of scarcity, and develop a strategy for the game that provides rational incentives for cooperation under a wide range of conditions. The second challenge is that the structure of a friend-to-friend overlay may reveal the users’ social relationships to an adversary monitoring the underlying network. To conceal their sensitive relationships from the adversary, the users must be able to communicate indirectly across the overlay in a way that resists monitoring and attacks by other participants. We address this second challenge by developing two new routing protocols that robustly deliver messages across networks with unknown topologies, without revealing the identities of the communication endpoints to intermediate nodes or vice versa. The protocols make use of a novel unforgeable acknowledgement mechanism that proves that a message has been delivered without identifying the source or destination of the message or the path by which it was delivered. One of the routing protocols is shown to be robust to attacks by malicious participants, while the other provides rational incentives for selfish participants to cooperate in forwarding messages. 3 Contents 1 Introduction 7 1.1 Background . 7 1.2 Goals . 9 1.3 Assumptions . 9 1.4 Requirements . 11 1.5 Thesis Statement . 11 1.6 Contributions . 12 1.7 Previously Published Work . 12 2 Related Work 13 2.1 Background . 13 2.2 Peer-to-Peer Networks . 15 2.3 Availability . 21 2.4 Anonymity . 27 2.5 Cooperation . 42 3 Analysis and Proposed Design 48 3.1 Evaluation of Existing Work . 48 3.2 Outline of the Proposed Design . 49 3.3 Rationale . 51 3.4 Design Strategy . 55 4 Cooperation in Distributed Networks 56 4.1 Game Theoretic Models of Cooperation . 56 4.2 Simulations . 60 4.3 Discussion . 62 4.4 Extensions to the Model . 64 4.5 Conclusion . 66 5 Unforgeable Acknowledgements 67 5.1 The Unforgeable Acknowledgement Mechanism . 67 5.2 Security . 69 5.3 Applicability . 70 4 5.4 Engineering Considerations . 71 5.5 Experimental Evaluation . 72 5.6 Related Work . 74 5.7 Discussion . 75 5.8 Conclusion . 77 6 Routing Protocols for Untrusted Networks 78 6.1 Assumptions and Terminology . 78 6.2 The Darknet Protocol . 79 6.3 Simulations . 85 6.4 Robustness . 91 6.5 Cooperation . 93 6.6 The Exu Protocol . 94 6.7 Conclusion . 101 7 Conclusions and Future Work 102 7.1 Summary of Results . 102 7.2 Discussion . 103 7.3 Future Work . 104 A The Simulation Framework 130 5 Acknowledgements Spending eight years on a puzzle is a selfish use of time, and of the many people I would like to thank for their support, advice, encouragement and inspiration during the time it has taken to write this thesis, there are some who first deserve an apology. Laura, Alan and Dan, I’m sorry for too often letting work get in the way. My thanks go first to my parents, who always encouraged me to be curious and to enjoy learning, although they may wish by now that they had emphasised the benefits of employment instead. I would also like to thank my colleagues at UCL and elsewhere – especially Denise, Piers, Manish, Torsten, Mo, Daniele, Toad and Matteo – for everything they taught me. To the friends in the CBR who pulled me through some difficult times and difficult haircuts – HB, Itch, Russell, Phil, Jimbo, Jamie, Mike – thank you. Lorena, I wouldn’t have understood the full horror of what it is to be a PhD student without our conversations. To the crew of the good ship LimeWire, now holed below the waterline, I offer my thanks for making me into a better coder and a worse drinker. I would also like to thank my supervisors, Steve Hailes, Cecilia Mascolo, Licia Capra, and above all Saleem Bhatti, without whose patience, generosity, insight, wise advice and good humour this long random walk would not have reached its destination. καλλιστη 6 Chapter 1 Introduction “Now, I been in jail when all my mail showed That a man can’t give his address out to bad company” – Bob Dylan, Absolutely Sweet Marie 1.1 Background Scientific research always exists within a social context. The goals of the present work – privacy and censorship- resistance – are social as well as technical concepts, and before defining them in technical terms we must first consider something of their social meaning. Censorship Censorship, narrowly defined, is the official examination and suppression of material that is to be published. However, in the context of Internet censorship, this definition immediately raises a number of questions. Can an automatic Internet filter be said to perform an official examination? Is denying access to published material from a particular location – such as a school, a library, or even an entire country – equivalent to prohibiting its publication? Can material that circulates between friends be said to have been published? Jansen [1] argues for a broader conception of censorship as “socially structured silence”, recognising that power relations may influence discourse in many ways. These include official and unofficial punishment and intimidation; social norms that deny certain individuals a public voice; institutional practices that limit the spread of information; and the power of markets and media owners to decide what is published. Whether such influence is active, as in the case of a government banning a newspaper, or passive, as in the case of a newspaper declining to cover a controversial story, the effect of censorship is to reinforce the relations of power that produce it. If censorship is an effect of power relations then we might suppose that it can be resisted by placing communication beyond the reach of power. Habermas [2] imagines an ideal speech situation where “no force except that of the better argument is exercised” and “all motives except that of the cooperative search for truth are excluded.” In this ideal situation, speakers seek universal consensus in “an atmosphere free of coercion or dependencies (inequalities) that would incline individuals toward acquiescence or silence” [3]. Habermas admits that this ideal is never achieved in practice, but it might be argued that even as an ideal, the notion of universal consensus implicitly deprecates dissent. We might also ask how consensus can be distinguished from mere compliance – Strauss [4] describes how the threat of persecution has led many authors to “write between the lines”, creating an appearance of agreement by concealing subversive messages within apparently conformist texts. Lyotard [5] rejects the goal of universal consensus in favour of a contestational model of discourse as a set of in- commensurable language games, where “every utterance should be thought of as a ‘move’ in a game.” For Lyotard, disagreement and dissent are enlivening rather than threatening; nevertheless, power can still intrude into language games in the form of terror: “eliminating, or threatening to eliminate, a player from the language game one shares 7 with him.” Thus, whether discourse is seen as the search for consensus or as a competitive and constantly evolving game, the influence of power relations cannot be ignored. Recent analyses of Internet censorship reveal a wide range of practices being used to control online discourse, from the simple filtering of Internet traffic to complex socio-technical systems of monitoring, self-regulation, intimidation and propaganda that blur the line between repressive and productive forms of power [6, 7, 8, 9, 10]. Clearly, contemporary censorship is not merely a matter of preventing access to information: it is a matter of shaping discourse in accordance with the demands of power. Surveillance and Privacy According to Murakami Wood [11], “Where we find purposeful, routine, systematic and focused attention paid to per- sonal details, for the sake of control, entitlement, management, influence or protection, we are looking at surveillance.” If censorship is socially structured silence, then surveillance is socially structured visibility. Foucault [12] uses the Panopticon, Bentham’s proposal for a circular prison in which the inmates are constantly and unobtrusively observed from a central watchtower [13], to illustrate the relationship between power and visibility. According to Foucault, those who are under surveillance internalise the power of the observer; control becomes self- control. “He who is subjected to a field of visibility, and who knows it, assumes responsibility for the constraints of power; he makes them play spontaneously upon himself; he inscribes in himself the power relation in which he simultaneously plays both roles; he becomes the principle of his own subjection.” In recent years, surveillance has moved beyond the walls of the institutions described by Foucault and has become ubiquitous.