In this paper, we propose a reasonable definition of predicate-hiding inner product encryption (IPE) in a public key setting, which we call inner product encryption with ciphertext conversion (IPE-CC), where original ciphertexts are converted to predicate-searchable ones by an helper in possession of a conversion key. We then define a notion of full security for IPE-CC, which comprises three security properties of being adaptively predicate- and attribute-hiding in the public key setting, adaptively (fully-)attribute-hiding against the helper, and usefully secure even against the private-key generator (PKG). We then present the first fully secure IPE-CC scheme, and convert it into the first fully secure symmetric-key IPE (SIPE) scheme, where the security is defined in the sense of Shen, Shi, Waters. All the security properties are proven under the decisional linear assumption in the standard model. The IPE-CC scheme is comparably as efficient as existing attribute-hiding (not predicate-hiding) IPE schemes. We also present a variant of the proposed IPE-CC scheme with the same security that achieves shorter public and secret keys. We
employ two key techniques, trapdoor basis setup, in which a new trapdoor is embedded in a public key, and multi-system proof technique, which further generalizes an extended dual system approach given by Okamoto and Takashima recently.
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