Towards efficient and accountable oblivious cloud storage
Qiumao Ma · Iowa State University Digital Repository (Iowa State University) · 2018
Due to the convenience and unprecedented cost effectiveness, more and more individuals and organizations have utilized cloud storage servers to host their data. However, because of security and privacy concerns, not all data can be outsourced without reservation. The concerns are rooted from the users' loss of data control from their hands to the cloud servers' premise and the infeasibility for them to fully trust the cloud servers. The cloud servers can be compromised by hackers, and they themselves may not be fully trustable. Though encryption helps to secure data, the server or the attacker who compromise the server is still able to infer private information from the user's access pattern. It is possible for an attacker to use the access pattern information to reconstruct the data query and infer the plaintext of the data. Hence, a large variety of schemes based on the oblivious RAM (ORAM) model have been proposed to allow a user to access the exported data while preserving user's data access pattern. Most of these research has focused on the communication efficiency improvement, but the storage efficiency has not received much attention. To host N data blocks, in general, the state-of-the-art ORAM constructions need the storage server to also store cN with c > 3 or O(N logN) dummy data blocks, which represents a huge storage overhead when N is large. In addition to the inefficiency in server storage, most of existing ORAM constructions incur O(logN) blocks or higher client-server communication cost. Though some recent work has reduced the cost to O(1) blocks by employing multiple non-colluding servers, the system could become vulnerable if some server does not follow the protocol completely. x To address the above limitations, we develop a series of new ORAM constructions, gradually towards a more practical and secure solution that can obliviously protect the data access pattern for users of cloud storage with more affordable storage, client-server communication, and server communication overheads. Specifically, this dissertation presents: SE-ORAM, which reduces server storage overhead to zero, but at the same time, incurs a client server communication cost of O(log2 N) blocks; Octopus ORAM, which incurs 0:34NB server storage overhead, and reduces client-server communication cost to three blocks for query and about 1:5 logN blocks for eviction per query; Three-server Octopus ORAM, an efficient and accountable multi-server ORAM, which incurs 0:3N B server storage overhead and reduces client-server communication cost to O(1) blocks, at the expense of server-server communication cost at O(logN) blocks per query. We have rigorously quantified and proved the security strengths of these constructions and demonstrated their performance efficiency through detailed analysis.