In this paper, we provide a secure and efficient encrypted keyword search scheme for multi-user data sharing. Specifically, a data owner outsources a set of encrypted files to an untrusted server, shares it with a set of users, and a user is allowed to search keywords in a subset of files that he is authorized to access. In the proposed scheme, (a) each user has a constant size secret key, (b) each user generates a constant size trapdoor for a keyword without getting any help from any party (e.g., data owner), independent of the number of files that he is authorized to search, and (c) for the keyword ciphertexts of a file, the network bandwidth usage (from the data owner to the server) and storage overhead at the server do not depend on the number of users that are authorized to access the file. We show that our scheme has data privacy and trapdoor privacy. While several recent studies are on secure keyword search for data sharing, we show that they either suffer from scalability issues or lack user privacy.
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