The Free Haven Project: Design and Deployment of an Anonymous Secure Data Haven
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The Free Haven Project: Design and Deployment of an Anonymous Secure Data Haven by Roger R. Dingledine Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degrees of Bachelor of Science in Computer Science and Engineering and Master of Engineering in Electrical Engineering and Computer Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2000 © Roger R. Dingledine, MM. All rights reserved. The author hereby grants to MIT permission to reproduce and distribute publicly paper and electronic copies of this thesis document in whole or in part. Author ................................... Department of Electrical Engineering andUCompuer Science May 22, 2000 7) A Certified by ................................... Ron Rivest Webster Professor of Computer Science and Engineering Thesis Supervisor Z7 Accepted by ............................. .............. g. .-. .... Arthur C. Smith Chairman, Department Committee on Graduate Students MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUL 2 7 2000 LIBRARIES -1 The Free Haven Project: Design and Deployment of an Anonymous Secure Data Haven by Roger R. Dingledine Submitted to the Department of Electrical Engineering and Computer Science on May 22, 2000, in partial fulfillment of the requirements for the degrees of Bachelor of Science in Computer Science and Engineering and Master of Engineering in Electrical Engineering and Computer Science Abstract The Free Haven Project aims to deploy a system for distributed data storage which is robust against attempts by powerful adversaries to find and destroy stored data. Free Haven uses a mixnet for com- munication, and it emphasizes distributed, reliable, and anonymous storage over efficient retrieval. We provide an outline of a formal definition of anonymity, to help characterize the protection that Free Haven provides and to help compare related services. We also provide some background from case law about anonymous speech and anonymous publication, and examine some of the ethical and moral implications of an anonymous publishing service. In addition, we describe a variety of attacks on the system and ways of protecting against these attacks. Some of the problems Free Haven addresses include providing sufficient accountability without sacrificing anonymity, building trust between servers based entirely on their observed behavior, and providing user interfaces that will make the system easy for end-users. Thesis Supervisor: Ron Rivest Title: Webster Professor of Computer Science and Engineering Contents 1 Introduction and Requirements 6 1.1 M otivation ............ ............... .. 6 1.2 Project Summary .......... ...................................... 7 1.3 Design Requirements .............................. 8 1.4 About this document . ............... ............... ..... 10 2 Defining Anonymity 11 2.1 Defining Privacy ...................................... 11 2.2 Agents and Operations ................................... 12 2.3 Linkability: Anonymity vs. Pseudonymity ........................ 14 2.4 Partial Anonymity vs. Full Anonymity .................... ...... 15 2.5 Characteristics of Communication Channels ....................... 16 2.6 What does it mean to 'break' an anonymous system? ............ ...... 17 2.7 Modeling the Adversary ............................ ...... 18 2.8 Toward a formal definition of anonymity ............. ............ 19 3 Related and Alternate Works 20 3.1 Anonymous Channels ....................... ............ 20 3.2 Publication Services ........................ ............ 25 3.3 Trust System s ........................................ 36 4 Applications, Legalities, and Ethics 38 4.1 Introduction and Assumptions ........ .......... .......... .. 38 4.2 Anonymous speech ....... .......... .......... .......... 39 4.3 Anonymous Publication .............. ............... ..... 43 4.4 Legal, yes - but Moral? .................................. 46 4.5 Conclusions ......... ............... ............... .. 55 4 5 System Design and Overview 57 5.1 System Summary .......... ...................................... 57 5.2 Supported Operations ................................... 60 5.3 Expiration .. ..... ..... ...... ..... .... ..... ..... ..... 63 5.4 Accountability and Redundancy .. ..... ..... .... ..... ..... ... 65 5.5 Composition of a Share ... ...... ..... ..... .... ..... ..... 68 5.6 Trading .. ..... ..... ...... ..... ..... .... ..... ..... 69 5.7 R eceipts . .... ..... .... .... .... ..... .... .... .... .... 72 5.8 Trust Networks ... ..... ...... ..... .... ..... ..... ..... 72 5.9 Directory Services . ..... ...... ..... ..... .... ..... ..... 78 5.10 U ser Interfaces .. ...... ...... ...... ...... ...... ...... 79 6 Communication Module and Protocols 82 6.1 M odule D esign .... ..... ..... ..... ..... ..... ..... ..... 82 6.2 Module Implementation .... ...... ..... .... ..... ..... ..... 84 7 Adversaries and Attacks 89 7.1 Attacks on the Communications Channel . ...... ...... ...... ..... 89 7.2 Attacks on the Infrastructure and Documents ...... ...... ...... .... 97 7.3 Attacks on the Trust System . ..... ..... ..... ...... ..... 101 8 Future Works 103 8.1 Communications Channels .. ...... ..... ..... .... ..... ..... 103 8.2 Publication Systems ..... ...... ..... ..... .... ..... ..... 108 8.3 Trust........... .... ........................................ 109 9 Conclusions 111 A Acknowledgements 113 B Anonymous Communications Channels 114 B .1 Proxy Services .. ..... ...... ..... ..... .... ..... ..... .. 114 B.2 Chaumian Mix-nets . ...... ...... ...... ...... ...... ..... 116 B.3 Remailers: SMTP Mix-nets ... ...... ...... ...... ...... ..... 117 B.4 Recent Mix-Net Designs .... ...... ...... ...... ...... ...... 120 B.5 Other Anonymous Channels ... ...... ..... .... ..... ..... .... 124 Bibliography 127 5 Chapter 1 Introduction and Requirements 1.1 Motivation The internet is moving in the direction of increasing freedom of information. National boundary lines are growing increasingly blurred. At the same time as a strong sense of global community is growing, technical advances have provided greatly increased bandwidth and an enormous amount of computing power and well-connected storage. However, the increases in speed and efficiency have not brought comparable increases in privacy and anonymity on the internet - indeed, governments and especially corporations are beginning to realize that they can leverage the internet to provide detailed information about the interests and behaviors of existing or potential customers. Court cases, such as the Church of Scientology's lawsuit against Johan Helsingius [42] or the more recent OpenDVD debate [55] (and subsequent arrest of DeCSS author Jon Lech Johansen), demonstrate that the internet currently lacks an adequate infrastructure for truly anonymous publication or distribution of documents or other data. Indeed, there are a number of other deeper motivations for the deployment of an anonymous publishing service such as Free Haven. Not only do we hope to assist those like Helsingius and Johansen, but we have the wider goals of pushing the world a few more steps in the direction of free and open information and communication. In Germany, Internet Service Providers such as AOL are liable for the content that passes across their systems [82]. Recent British legislation threatens to make citizens responsible for the content of encrypted documents that they're holding, even if they don't possess the ability to read these documents [74]. Such restrictions on the free flow of information, however, are already being attacked: for example, American hackers are attempting to break holes in China's "Great Firewall" to allow Chinese citizens access to Western media [11]. In addition to such revolutionary actions, there are a wide range of activist projects which employ the internet for publicity but focus on helping real people in the real world. Such projects include 6 The Free Haven Project Roger Dingledine Pirate Radio [29], a loose confederation of radio operators joined in the belief that ordinary citizens can regulate the airwaves more efficiently and more responsibly than a government organization; as well as mutual aid societies such as Food Not Bombs! [45], an organization which "serves free food in public places to dramatise the plight of the homeless, the callousness of the system and our capacity to solve social problems through our own actions without government or capitalism." By providing tools to enable safer and more reliable communication for organizations fighting for increased rights of individuals rather than nations or corporations, as well as strengthening the capabilities of political dissidents and other individuals to speak out anonymously about their situations, the members of the Free Haven Project hope to help pave the way to a modern society where freedom of speech and freedom of information are integral parts of everyday life. 1.2 Project Summary The Free Haven Project intends to deploy a system which provides a good infrastructure for anony- mous publication. Specifically, this means that the publisher of a given document should not be known; that clients requesting the document should not have to identify themselves to anyone; and that the current location of the document should not be known. Additionally, it would be preferable to limit the number of opportunities where an outsider can show that a given document passed through a given computer. A more thorough examination