Design and Analysis Methods for Privacy Technologies
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Arenberg Doctoral School of Science, Engineering & Technology Faculty of Engineering Department of Electrical Engineering (ESAT) Design and analysis methods for privacy technologies Carmela Troncoso Dissertation presented in partial fulfillment of the requirements for the degree of Doctor in Electrical Engineering April 2011 Design and analysis methods for privacy technologies Carmela Troncoso Jury: Prof. Dr. Ir. Ann Haegemans, chairman Prof. Dr. Ir. Adhemar Bultheel, acting chairman Prof. Dr. Ir. Bart Preneel, promotor Dissertation presented in partial Prof. Dr. Ir. Claudia Diaz, promotor fulfillment of the requirements for Prof. Dr. Ir. Geert Deconinck the degree of Doctor Prof. Dr. Ir. Frank Piessens in Electrical Engineering Prof. Dr. Nikita Borisov (University of Illinois at Urbana-Champaign) Dr. George Danezis (Microsoft Research Cambridge) April 2011 © Katholieke Universiteit Leuven – Faculty of Engineering Arenbergkasteel, B-3001 Leuven-Heverlee, Belgium Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaar gemaakt worden door middel van druk, fotocopie, microfilm, elektronisch of op welke andere wijze ook zonder voorafgaande schriftelijke toestemming van de uitgever. All rights reserved. No part of the publication may be reproduced in any form by print, photoprint, microfilm or any other means without written permission from the publisher. Legal depot number D/2011/7515/52 ISBN number 978-94-6018-350-8 Acknowledgements People have always told me that writing a thesis is a solo journey. Now I am at the end of the road and when I look back there were hard moments, but I have felt everything but alone in this quest. First of all I want to thank my advisors Bart Preneel and Claudia Diaz, who believed in me from the first day and let me know they trusted me every day of my PhD. I would like to thank Bart for his support; for (almost) never saying no to anything that I asked for no matter how crazy the idea was (and I have had a fair amount of crazy ideas); and for telling me something that changed my life: “Shit happens, you’ll deal with it”. Thanking Claudia for all that she did for me in a few words is very difficult. There are many good advisors out there able to teach their students how to do research, but friends that feel like family there are very few. I am so lucky that I found both of them in one person. I am also very grateful to Prof. Frank Piessens, Prof. Geert Deconinck, Prof. Nikita Borisov and Dr. George Danezis for serving as jury members, and giving me many suggestions to improve this dissertation. I want to thank Prof. Ann Haegemans and Prof. Adhemar Bultheel for chairing the jury. I would not have written this thesis if I had not had the luck of sharing an office not only with Claudia, but also with George. When I knew nothing about security nor privacy he took his time to guide me, always giving me new opportunities to learn. I also thank him for introducing me to the fascinating world of Bayesian inference: the gift that keeps on giving, the Angry Nights, the Rollerkillers, the Filthy Dragon, Indigo’s bagels, for teaching me my first words in Greek,. but above everything I would like to thank him for making me realize why it is worthy to work on privacy. George, I shall not forget: “Girls come and go, traffic analysis is forever not for Christmas.” I would also like to thank all of my co-authors for having the patience to work with me, letting me learn from them, and having made each new paper not only a nice academic experience but also fun. A very special thanks goes to Benedikt, a friend inside and outside of ESAT, always there to support me not only at work but also in life making his home my home (with better breakfast). I cannot imagine my i life in Leuven without sharing dinner, a washing machine, a car, a bbq, garden furniture,. with him. Dude, when are you coming this summer? Not only my co-authors have had an influence on my research. A big thanks goes to all the people that have been part of Cosic during these years and created the best working environment for me. In particular thanks to Josep, the best office mate ever; to Seda and Danny, always eager to help and take care of the little things that make my life so much easier; to Pela, that made the damn paperwork even look nice when I was having a coffee at her desk; and to Fonsi, Orr, Markulf, Elena, Basti, Emily, K˚are,Ero, for so many coffees, laughters, beers, dinners, game evenings, climbing hours, etc. I would also like to thank my office and lunch mates in Vigo for making going to work a pleasant experience in my last months of writing. A more than special thanks go to Elketje. I would need to write a whole new book to really express all that I am thankful for, but in few words: thank you not only for letting me see the different sides of Elke, and teach me that there is more than one point of view in life and thus there is not only one definition for right and wrong; but more than anything for being there every day to remind me. Then there are all those people that do not know a word about security or maths, but that have been as important for my PhD as those whom I worked with. The warmest thanks to Alba and OneC de Montalvo, for never letting me give up, for making pancakes when they were needed, for understanding how cool it would be to introduce a car in a DVD, and for always being ready to give up a bit of their happiness to help me. Thanks to Mil Amigos, each and one of them, because no matter how far they are they always sound near and with them I always know that whatever it is, it is going to be “chichi!”. Thanks to Fra and Sarah for reminding me I actually enjoy going out, and having a life. I would never have written this thesis without Hayd´eeand Who, always there to calm me down on the other side of the phone. And a very special thanks to Eva, for always encouraging me to follow my dreams even when they were not aligned with hers. I would not have a PhD without her. Finally, I am deeply thankful to my family. Thanks to my parents for building a safety net such that I could run around catching dreams without fear to fall, and to my sister for being green and listen to me and always being so keen in telling me I am overreacting. I also thank my aunt Arita, my godmother Celia, and the whole Quiroga clan for always have their door open to me, ready to give me exactly what I need without ever making questions. To all those I have mentioned, and all those that are not mentioned but have been by my side all these years: Thank you for being a part of this! I would like to acknowledge the K.U.Leuven, the Fundaci´onPedro Barri´ede la Maza, and the Fund for Scientific Research in Flanders (FWO) for funding my research. Abstract As advances in technology increase data processing and storage capabilities, the collection of massive amounts of electronic data raises new challenging privacy concerns. Hence, it is essential that system designers consider privacy requirements and have appropriate tools to analyze the privacy properties offered by new designs. Nevertheless, the privacy community has not yet developed a general methodology that allows engineers to embed privacy-preserving mechanisms in their designs, and test their efficacy. Instead, privacy-preserving solutions are designed and analyzed in an ad hoc manner, and hence it is difficult to compare and combine them in real-world solutions. In this thesis we investigate whether general methodologies for the design and analysis of privacy-preserving systems can be developed. Our goal is to lay down the foundations for a privacy engineering discipline that provides system designers with tools to build robust privacy-preserving systems. We first present a general method to quantify information leaks in any privacy- preserving design that can be modeled probabilistically. This method allows the designer to evaluate the degree of privacy protection provided by the system. Using anonymous communication systems as case study we find that Bayesian inference and the associated Markov Chain Monte Carlo sampling techniques form an appropriate framework to evaluate the resistance of these systems to traffic analysis. The Bayesian approach provides the analyst with a neat procedure to follow, starting with the definition of a probabilistic model that is inverted and sampled to estimate quantities of interest. Further, the analysis methodology is not limited to specific quantities such as “who is the most likely receiver of Alice’s message?,” but can be used to answer arbitrary questions about the entities in the system. Finally, our methodology ensures that systematic biases in information analysis are avoided and provides accurate error estimates. In the second part of this thesis we tackle the design of privacy-preserving systems, using pay-as-you-drive applications as case study. We propose two pay-as-you-drive architectures that preserve privacy by processing personal data locally to the users, and only communicating billing information to the provider. v Local processing enhances privacy, but may be detrimental to other security properties such as service integrity (e.g., the provider has access to less data when verifying the correctness of the bill).