*_Notice and Invitation_* Oral Defense of Doctoral Dissertation The Volgenau School of Engineering, George Mason University Meixing Le Bachelor of Science, Fudan University, 2004 Master of Science, Fudan University, 2007 *Secure Cooperative Data Access in Multi-Cloud Environment* Wednesday, May 1^st 2013, 9:00AM Research Hall, Room 401 All are invited to attend. *_Committee_* Dr. Sushil Jajodia, Co-Chair Dr. Krishna Kant, Co-Chair Dr. Angelos Stavrou Dr. Massimiliano Albanese Dr. Duminda Wijesekera *_Abstract_* To implement desired bushiness services, certain queries against useful information need to be executed in a multi-cloud environment where the data is owned and managed by multiple collaborating enterprises. In such cases, authorization rules over the joins of relational data are given so that parties selectively share information with each other. Based on that, we define the notion of rule consistency and devise an algorithm to ensure it even in case of rule changes. In addition, not all the authorization rules can be safely enforced among existing parties. To that end, we propose a bottom-up algorithm to systematically check the rule enforceability for each rule and a mechanism to generate a query execution plan that is consistent with the authorization rules for each incoming authorized query. For unenforceable rules, trusted third parties are needed and it is important to minimize the interaction with third parties. We define cost models and formulate the minimization problem. We show the hardness of this problem and propose efficient algorithm to search solutions. With extensive simulation evaluations, the results show the effectiveness of the approach. A copy of this doctoral dissertation is on reserve at the Johnson Center Library.