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Cryptography for Efficiency: New Directions In Abstract of \Cryptography for Efficiency: New Directions in Authenticated Data Structures" by Charalampos Papamanthou, Ph.D., Brown University, May 2011. Cloud computing has emerged as an important new computational and storage medium and is increasingly being adopted both by companies and individuals as a means of reducing operational and maintenance costs. However, remotely-stored sensitive data may be lost or modified and third-party computations may not be performed correctly due to errors, op- portunistic behavior, or malicious attacks. Thus, while the cloud is an attractive alternative to local trusted computational resources, users need integrity guarantees in order to fully adopt this new paradigm. Specifically, they need to be assured that uploaded data has not been altered and outsourced computations have been performed correctly. Tackling the above problems requires the design of protocols that, on the one hand, are provably secure and at the same time remain highly efficient, otherwise the main purpose of adopting cloud computing, namely efficiency and scalability, is defeated. It is therefore essential that expertise in cryptography and efficient algorithmics be combined to achieve these goals. This thesis studies techniques allowing the efficient verification of data integrity and computations correctness in such adversarial environments. Towards this end, several new authenticated data structures for fundamental algorithmics and computation problems, e.g., hash table queries and set operations, are proposed. The main novelty of this work lies in employing advanced cryptography such as lattices and bilinear maps, towards achieving high efficiency, departing from traditional hash-based primitives. As such, the proposed techniques lead to efficient solutions that introduce minimal asymptotic overhead and at the same time enable highly-desirable features such as optimal verification mechanisms and par- allel authenticated data structures algorithms. The small asymptotic overhead does translate into significant practical savings, yielding efficient protocols and system prototypes. Cryptography for Efficiency: New Directions in Authenticated Data Structures by Charalampos Papamanthou B.Sc., Applied Informatics, University of Macedonia, 2003 M.Sc., Computer Science, University of Crete, 2005 M.Sc., Computer Science, Brown University, 2007 A dissertation submitted in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in the Department of Computer Science at Brown University Providence, Rhode Island May 2011 c Copyright 2011 by Charalampos Papamanthou This dissertation by Charalampos Papamanthou is accepted in its present form by the Department of Computer Science as satisfying the dissertation requirement for the degree of Doctor of Philosophy. Date Roberto Tamassia, Director Recommended to the Graduate Council Date Michael T. Goodrich, Reader University of California, Irvine Date Anna Lysyanskaya, Reader Date Franco P. Preparata, Reader Approved by the Graduate Council Date Peter M. Weber Dean of the Graduate School iii Vita Charalampos Papamanthou was born in Trikala, Greece, 29 years ago. Right after graduation from high school, he began his college studies in Thessaloniki, Greece, receiving his bachelor's degree in Applied Informatics from the University of Macedonia in 2003. He then traveled south to pursue a master's degree in Computer Science at the beautiful island of Crete. There, and under the Mediterranean sun, he also did research at the Foundation for Research and Technology Hellas. Upon completion of his studies at the University of Crete in 2005, he decided to cross the Atlantic and move to Providence, Rhode Island, in order to attend Brown University for graduate school. At Brown, he received both his master's and doctoral degrees in Computer Science in 2007 and 2011 respectively. He was also the recipient of the Kanellakis and the van Dam fellowships. While at graduate school, he spent two summers at the West Coast, interning at Intel Research and Microsoft Research. His research interests are in computer security, applied cryptography and in the design and analysis of algorithms. Beginning summer 2011, he will be joining the University of California at Berkeley to work as a postdoctoral researcher at the Computer Science Division. iv Preface Cloud computing has emerged as an important new computational and storage medium and is increasingly being adopted both by companies and individuals as a means of reducing operational and maintenance costs. However, remotely-stored sensitive data may be lost or modified and third-party computations may not be performed correctly due to errors, op- portunistic behavior, or malicious attacks. Thus, while the cloud is an attractive alternative to local trusted computational resources, users need integrity guarantees in order to fully adopt this new paradigm. Specifically, they need to be assured that uploaded data has not been altered and outsourced computations have been performed correctly. Tackling the above problems requires the design of protocols that, on the one hand, are provably secure and at the same time remain highly efficient, otherwise the main purpose of adopting cloud computing, namely efficiency and scalability, is defeated. It is therefore essential that expertise in cryptography and efficient algorithmics be combined to achieve these goals. This thesis studies techniques allowing the efficient verification of data integrity and computations correctness in such adversarial environments. Towards this end, several new authenticated data structures for fundamental algorithmics and computation problems, e.g., hash table queries and set operations, are proposed. The main novelty of this work lies in employing advanced cryptography such as lattices and bilinear maps, towards achieving high efficiency, departing from traditional hash-based primitives. As such, the proposed v techniques lead to efficient solutions that introduce minimal asymptotic overhead and at the same time enable highly-desirable features such as optimal verification mechanisms and par- allel authenticated data structures algorithms. The small asymptotic overhead does translate into significant practical savings, yielding efficient protocols and system prototypes. vi Acknowledgments Many individuals contributed to the outcome of this beautiful educational journey at Brown University. First and foremost, I deeply thank my thesis advisor, Roberto Tamassia, who guided me through the challenging path of graduate school. Roberto's vast experience in research, combined with his kindness, smile and sincerity, taught me how to produce high-quality work with a positive attitude, always being precise, objective and very self-critical. His efficient quest for perfection, his work ethic, as well as his constructive feedback were vital in shaping not only my research philosophy, but also my daily presence and interactions in an academic environment. Finally, Roberto's advice on personal matters and academics has been really invaluable and was always promptly and generously provided, whenever needed. I could not have hoped for a better advisor. Second, I am grateful to Franco P. Preparata, with whom I closely collaborated during my first two years at Brown. Franco was the first faculty member I met as soon as I arrived in Providence, back in 2005. Having known Franco for six years now, I am still amazed by his seemingly endless knowledge of Computer Science, his high integrity, and his loyalty to his colleagues. I thank him for the so many technical and political discussions we had, his meticulously prepared lectures on parallel algorithms and computational biology, and for the provably correct advice he would always provide at the right time. Also, I would like to thank his wife, Rosa Maria, for inviting me multiple times for dinner at their place. vii Admittedly these have been the most original and tasteful Italian dinners ever! I would also like to thank the other members of my committee, Michael T. Goodrich and Anna Lysyanskaya. Michael has been a great collaborator, always encouraging new ideas and a diverse research agenda. He provided excellent feedback on the final text of this thesis. Anna taught me foundations of cryptography, through an engaging introductory class and through the crypto reading group. Her presence in the department and my interactions with her greatly influenced the research path of this dissertation. Also, many thanks to Nikos Triandopoulos, who, apart from a close friend, has been a reliable colleague, always eager to carefully listen to all my ideas and concerns. Many results in this dissertation have been the outcome of a great deal of fruitful discussions and long technical meetings with him. Finally, I would like to thank Alptekin K¨up¸c¨u,C. Chris Erway, Bernardo Palazzi, Alexander Heitzmann, Olya Ohrimenko and Danfeng Yao for the work we did together on topics related to this thesis, as well as Petros Maniatis and Seny Kamara for being my internship mentors at Intel Research and Microsoft Research respectively. The Brown CS faculty, and in particular the professors I interacted with mostly, namely, John Savage, Claire Mathieu, Philip Klein, Tom Doeppner, Pascal Van Hentenryck, Rodrigo Fonseca and Eli Upfal, have been extraordinary. Their persistent dedication to high-quality research and teaching nurtured an inspiring and challenging environment for every graduate student in the department. Also, everyday life in the CIT would not have come easy had it not been for the Brown
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