Searching on Encrypted Data

Abstract

Searchable encryption allows a user to outsource encrypted data to a remote server, whilst preserving the user’s ability to locate specific data items within the encrypted data that satisfy some query. In its simplest form, searchable encryption allows a user to locate all data items that contain a particular keyword.In this thesis, we analyse searchable encryption schemes and assess their suitability for various scenarios occurring in the real world. Despite the existence of practical searchable encryption schemes in the literature, there is limited evidence of their de- ployment. We discuss issues that we consider to be inhibiting the widespread adoption of searchable encryption. This work aims to present searchable encryption as a useable technology and, by analysing the efficiencies of the various schemes within the different scenarios, we intend to make the design of new real-world searchable encryption protocols an easier task.We also present two new searchable encryption schemes. A number of searchable encryption schemes have been proposed that are secure in the presence of a semi-honest server, which may deviate from the protocol in order to conserve its own resources. In existing schemes, the search queries a user can perform are not particularly expressive. We use techniques from publicly verifiable computation to build a searchable encryption scheme that can evaluate more expressive queries in a verifiable manner; that is, the user is able to verify whether or not the server has computed the search honestly.Our second construction allows users with different access rights to the data to receive search results which are dependent on their access rights. We call this type of scheme searchable encryption with multi-level access. Most existing searchable encryp- tion with multi-level access schemes are built using attribute based encryption, a form of public key cryptography. Symmetric key cryptography uses simpler and easier to im- plement primitives, compared to its public-key counterpart. We present a construction that extends a well-known searchable symmetric encryption scheme to support users searching the data at different access levels