An Improvement to a Multi-Client Searchable Encryption Scheme for Boolean Queries

The migration of e-health systems to the cloud computing brings huge benefits, as same as some security risks. Searchable Encryption(SE) is a cryptography encryption scheme that can protect the confidentiality of data and utilize the encrypted data at the same time. The SE scheme proposed by Cash et al. in Crypto2013 and its follow-up work in CCS2013 are most practical SE Scheme that support Boolean queries at present. In their scheme, the data user has to generate the search tokens by the counter number one by one and interact with server repeatedly, until he meets the correct one, or goes through plenty of tokens to illustrate that there is no search result. In this paper, we make an improvement to their scheme. We allow server to send back some information and help the user to generate exact search token in the search phase. In our scheme, there are only two round interaction between server and user, and the search token has (n - 1) x vertical bar I-w1 vertical bar elements, where n is the keywords number in query expression, and vertical bar Iw1 vertical bar is the minimum documents number that contains one of keyword in query expression, and the computation cost of server is (n - 1) x vertical bar Iw1 vertical bar modular exponentiation operation.