Alptekin Küpçü

of “Efficient Cryptography for the Next Generation Secure Cloud” by Alptekin Küpçü, Ph.D., Brown University, May 2010. Peer-to-peer (P2P) systems, and client-server type storage and computation outsourcing constitute some of the major applications that the next generation cloud schemes will address. Since these applications are just emerging, it is the perfect time to design them with security and privacy in mind. Furthermore, considering the highchurn characteristics of such systems, the cryptographic protocols employed must be efficient and scalable. This thesis shows that cryptography can be used to efficiently and scalably provide security and privacy for the next generation cloud systems. We start by describing an efficient and scalable fair exchange protocol that can be used for exchanging files between participants of a P2P file sharing system. In this system, there are two central authorities that we introduce: the arbiter and the bank. We then try distributing these entities to reduce trust assumptions and to improve performance. Our work on distributing the arbiter leads to impossibility results, whereas our work on distributing the bank leads to a more general cloud computation result showing how a boss can employ untrusted contractors, and fine or reward them. We then consider cloud storage scenario, where the client outsources storage of her files to an untrusted server. We show how the client can challenge the server to prove that her file is kept intact, even when the files are dynamic. Next, we provide an agreement protocol for a dynamic message, where two parties agree on the latest version of a message that changes over time. We then apply this agreement protocol to the cloud storage setting and show how a judge can arbitrate between the client and the server officially based on the agreed-upon message and the proof sent by the server. Lastly, we show that all our solutions are efficient and scalable by presenting results from the cryptographic library we implemented. Efficient Cryptography for the Next Generation Secure Cloud by Alptekin Küpçü B.S., Bilkent University, 2004 M.Sc., 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 2010 c © Copyright 2010 by Alptekin Küpçü This dissertation by Alptekin Küpçü 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 Prof. Anna Lysyanskaya, Advisor Recommended to the Graduate Council Date Prof. Yevgeniy Dodis, Reader NYU Date Prof. John Jannotti, Reader Date Prof. Roberto Tamassia, Reader Approved by the Graduate Council Date Sheila Bonde Dean of the Graduate School iii