Efficient Conjunctive Keyword-Searchable Encryption

A keyword-searchable encryption scheme allows a user with a "trapdoor" for a keyword to efficiently retrieve some of encrypted data containing the specific keyword over a remote server. The scheme for keyword-searchable encryption is considered as one of crucial building blocks that solves the security problems of privacy and data confidentiality in many settings, such as outsourced database systems and mail (or file) servers. However, most existing schemes support only a single keyword for searching, but do not allow for boolean combinations of keywords. It makes the use of schemes impractical in real applications. To address this problem, we propose an efficient construction for conjunctive keyword-searchable encryption, in which the size of trapdoors is almost same as that for searching a single keyword. Our construction is proven secure against adaptive chosen-keyword attacks in the random oracle model under the external co-Diffie-Hellman assumption. Compared to previous works, our construction has much better performance in terms of both computational and communication cost.