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Privacy preserving search in large encrypted databases

Tahir, S. (2018). Privacy preserving search in large encrypted databases. (Unpublished Doctoral thesis, City, University of London)


The Cloud is an environment designed for the provision of on-demand resource sharing and data access to remotely located clients and devices. Once data is outsourced to the Cloud, clients tend to lose control of their data thus becoming susceptible to data theft. To mitigate/ reduce the chances of data theft, Cloud service providers employ methods like encrypting data prior to outsourcing it to the Cloud. Although this increases security, it also gives rise to the challenge of searching and sifting through the large amounts of encrypted documents present in the Cloud.

This thesis proposes a comprehensive framework that provides Searchable Encryption-as-a-Service (SEaaS) by enabling clients to search for keyword(s) over the encrypted data stored in the Cloud. Searchable Encryption (SE) is a methodology based on recognized cryptographic primitives to enable a client to search over the encrypted Cloud data. This research makes five major contributions to the field of Searchable Encryption:

The first contribution is that the thesis proposes novel index-based SE schemes that increase the query effectiveness while being lightweight. To increase query effectiveness this thesis presents schemes that facilitate single-keyword, parallelized disjunctive-keyword (multi-keyword) and fuzzy-keyword searches.

The second contribution of this research is the incorporation of probabilistic trapdoors in all the proposed schemes. Probabilistic trapdoors enable the client to hide the search pattern even when the same keyword is searched repeatedly. Hence, this quality allows the client to resist distinguishability attacks and prevents attackers from inferring the search pattern.

The third contribution is the enumeration of a “Privacy-preserving” SE scheme by presenting new definitions for SE; i.e., keyword-trapdoor indistinguishability and trapdoor index indistinguishability. The existing security definitions proposed for SE did not take into account the incorporation of probabilistic trapdoors hence they were not readily applicable to our proposed schemes; hence new definitions have been studied.

The fourth contribution is the validation that the proposed index-based SE schemes are efficient and can be deployed on to the real-world Cloud offering. The proposed schemes have been implemented and proof-of-concept prototypes have been deployed onto the British Telecommunication’s Cloud Server (BTCS). Once deployed onto the BTCS the proof-of-concept prototypes have been tested over a large real-world speech corpus.

The fifth contribution of the thesis is the study of a novel homomorphic SE scheme based on probabilistic trapdoors for the provision of higher level of security and privacy. The proposed scheme is constructed on a Partially Homomorphic Encryption Scheme that is lightweight when compared to existing Fully Homomorphic-based SE schemes. The scheme also provides non-repudiation of the transmitted trapdoor while eliminating the need for a centralized data structure, thereby facilitating scalability across Cross-Cloud platforms.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics > QA75 Electronic computers. Computer science
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering
School of Mathematics, Computer Science & Engineering > Engineering > Electrical & Electronic Engineering
[img] Text - Accepted Version
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