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Signature Schemes and Applications to Cryptographic Protocol Design by Anna Lysyanskaya Submitted to the Department of Electrical Engineering and Computer Science

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Signature Schemes and Applications to Cryptographic Protocol Design by Anna Lysyanskaya Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree )' MASSACHUSETTS INSTITUTE OF TECHNOLOGY Doctor of Philosophy NOV 18 2dO2 at the LIBRARIES MASSACHUSETTS INSTITUTE OF TECHNOLOGY BARKER September 2002 @ Massachusetts Institute of Technology 2002. All rights reserved. Author ................................ Department of Electrical Engineering and Computer Science September 4, 2002 Certified by................... .. ...... .. .. .. ..... Ronald L. Rivest Viterbi Professor of Computer Science Thesis Supervisor Accepted by ......... - . ............ Arthur C. Smith Chairman, Department Committee on Graduate Students Signature Schemes and Applications to Cryptographic Protocol Design by Anna Lysyanskaya Submitted to the Department of Electrical Engineering and Computer Science on September 4, 2002, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract Signature schemes are fundamental cryptographic primitives, useful as a stand-alone appli- cation, and as a building block in the design of secure protocols and other cryptographic objects. In this thesis, we study both the uses that signature schemes find in protocols, and the design of signature schemes suitable for a broad range of applications. An important application of digital signature schemes is an anonymous credential sys- tem. In such a system, one can obtain and prove possession of credentials without revealing any additional information. Such systems are the best means of balancing the need of indi- viduals for privacy with the need of large organizations to verify that the people they interact with have the required credentials. We show how to construct an efficient anonymous cre- dential system using an appropriate signature scheme; we then give an example of such a signature scheme. The resulting system is the first one with satisfactory communication and computation costs. The signature scheme we use to construct an anonymous credential system is of inde- pendent interest for use in other protocols. The special property of this signature scheme is that it admits an efficient protocol for a zero-knowledge proof of knowledge of a signature. Further, we consider the question of revocation of signatures. We obtain an efficient revocation scheme. This has immediate consequences for revocation of credentials in our credential system. We explore other uses for signature schemes as building blocks for designing crypto- graphic objects and secure protocols. We give a unique signature scheme which has impli- cations for verifiable random functions and for non-interactive zero-knowledge proofs. Finally, we consider the use of signatures for implementing a broadcast channel in a point-to-point network. It was previously shown that while broadcast was impossible with- out computational assumptions in a point-to-point network where one-third or more nodes exhibited adversarial behavior, using an appropriate set-up phase and a signature scheme, the impossibility could be overcome. We show that the situation is more complex than was previously believed. We consider the composition of protocols in this model, and discover severe limitations. We also show how to augment the model to overcome these limitations. Thesis Supervisor: Ronald L. Rivest Title: Viterbi Professor of Computer Science 3 4 Acknowledgments My experience as a graduate student at MIT has been amazing. It would not have been the same without my mentors and friends. First and foremost, I would like to thank my advisor Ron Rivest. Ron has been a perfect advisor for me, and I feel that I have been truly privileged to work under his supervision. I am grateful to him for giving me direction and confidence, for being patient, encouraging, and kind, and for everything that I learned from him. Silvio Micali has had a tremendous impact on the way that I perceive research and life in general. I am grateful to him for the interest that he has taken in helping me develop both as a researcher and as a teacher, and for his overflowing energy and enthusiasm. My sincere thanks to Shafi Goldwasser. Shafi has always taken an interest in my work, and I am grateful to her for her encouragement, kindness, and patience. I am very grateful to Madhu Sudan for always being friendly, open and helpful. I also thank him for inviting me to take part in organizing the theory seminar at MIT. My gratitude also goes to Charles Leiserson, who was my graduate counselor and helped me a great deal, always having some valuable insight and advice for me. The theory group at MIT's Lab for Computer Science has been a nurturing and stimulating environment. I am grateful to Leo Reyzin, whom I can always rely upon for finding an approach to any problem, technical or otherwise, and whose friendship I greatly value. I thank Yevgeniy Dodis for his sincerity and friendship, and for giving me many technical insights. Many thanks to Alantha Newman for being my friend and for her sincere help and support throughout my time at MIT. I am grateful to Tal Malkin who befriended me the moment that I arrived. Eric Lehman deserves special thanks for being my friend, as well as the Theory Jeopardy Tsar. I am grateful to Stas Jarecki, Moses Liskov, Tal Malkin, Chris Peikert, Zully Ramzan, Leo Reyzin, Amit Sahai and Adam Smith for fruitful research collaborations. I am also very grateful to Moses Liskov for sharing in my first teaching experience. Chocolate greatly helped! Thank you, Be Blackburn, for being the administrative mom of the theory group and for taking care of me! I am also grateful to Marilyn Pierce, the goddess of EECS. With grief over the events of September 11, 2001 and his tragic death, I thank Danny Lewin, who was my first officemate and took care of me while I was a clueless first-year. My two summers at IBM Zurich (1999 and 2000) were unforgettable. I am very grateful to Christian Cachin for making this experience possible, and to him, Victor Shoup and Jan Camenisch for hosting me there. Discussions with Victor have bene- fitted me greatly, and I am very grateful to him. Jan has been a great collaborator and a wonderful friend. The work I did jointly with Jan makes up a substantial part of my research experience, and part of this thesis is based on it. It has been a great pleasure to work with him. I am also happy to have collaborated with Klaus Kursawe. I am very grateful to the IBM T.J.Watson's Crypto group who hosted me during the summer of 2001. My thanks go to Tal Rabin, as well as Ran Canetti, Rosario 5 Gennaro, Shai Halevi, Nick Howgrave-Graham, and Yehuda Lindell, for giving me such a fun experience and for working with me on research problems. Part of this thesis is based on joint work with Tal and Yehuda. Many thanks to Peter Winkler, my mentor at Bell Laboratories, where I worked for two very happy summers (1997 and 1998), for teaching me to work on research problems, for advising me about life, and for all the great stories and beautiful puzzles he told me. I am very grateful to my Smith College undergraduate advisors, Joe O'Rourke and Phyllis Cassidy, for sending me on this path. I worked on my first research project under Joe's supervision, and I would like to thank him for his encouragement and enthusiasm. I would like to thank my roommates, Maja Razlog and Alice Wang, for putting up with me all this time and for being my closest friends. My love and gratitude to my parents, Nadezhda and Alexander Lysyansky, for everything-everything-everything. (Funding for my graduate studies has been provided by an NSF graduate research fellowship, Lucent Technologies graduate research program for women, and DARPA, Merrill Lynch and NTT grants given to Ron Rivest.) 6 Contents 1 Introduction 11 1.1 Digital Signature Schemes . ............ ........... 12 1.1.1 Use of Signatures in Protocols ............ ...... 14 1.1.2 Revocation of Signatures ... .................. 16 1.1.3 Unique Signatures ........................ 17 1.1.4 Other Applications of Signatures in Protocols ......... 18 1.2 Privacy of D ata .............................. 18 1.2.1 Review of Existing Technologies .. ............ ... 20 1.2.2 Identity and Anonymity in the Electronic Setting ....... 22 1.3 Summary of This Thesis ............ ............ 23 2 Preliminaries 25 2.1 N otation ........................ .......... 25 2.2 Definitions of Security for Signature Schemes .............. 26 2.3 Commitment Schemes ..................... ..... 27 2.3.1 Trapdoor commitments ............. ......... 28 2.4 Zero-Knowledge Proofs ...... ......... ........ ... 29 2.5 Proofs of Knowledge . ............. ............ 31 2.5.1 Concurrent Composition ..... ....... ........ 33 2.6 Protocols for Models with Public Parameters .............. 36 2.6.1 Concurrent Zero-knowledge Using Trapdoor Commitments .. 37 3 Anonymous Credential System 41 3.1 Formal Definitions and Requirements ... ............... 43 3.1.1 Definition of a Secure System Conforming to an Ideal-World Functionality ....... .................... 43 3.1.2 Ideal Basic Credential System Functionality ....... ... 46 3.2 Constructing a Basic Scheme using Signatures and Commitments . 47 3.2.1 Signing a Committed Value .... ............ ... 48 3.2.2 Basic Anonymous Credential System ...... ........ 49 3.2.3 Proof of Security ......................... 50 3.2.4 Discussion and Efficiency Analysis ................ 52 3.3 Building in Additional Desirable
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