Public Key Encryption with Hierarchical Authorized Keyword Search

Public key encryption with keyword search (PEKS), which was introduced by Boneh et al. at EUROCRYPT’ 04, is a breakthrough approach to searching encrypted data under a public key setting. In this cryptographic primitive, senders can generate searchable ciphertexts for specific keywords to be retrieved from a given document; receivers can generate corresponding trapdoors for search by using their private keys. Recently, Jiang et al. (ACISP’ 16) proposed an improved PEKS scheme called public key encryption with authorized keyword search (PEAKS); this scheme enables authorized users to generate trapdoors for specific sets of keywords even if these users do not have access to the private key. Unfortunately, authorized users cannot delegate this power to other unauthorized users because the authorization in PEAKS is insufficiently flexible; therefore, this scheme is not suitable for enterprise scenarios in general. In this work, we introduce a novel cryptographic primitive called public key encryption with hierarchical authorized keyword search (PEHAKS) to solve this problem. In contrast to PEAKS, the proposed primitive enables authorized users to further hierarchically delegate their power of generating trapdoors to unauthorized users. We formally define the system model of PEHAKS under a multikeyword setting, and the security requirements are designed to withstand attacks in a real scenario. Furthermore, we propose a provably secure scheme using the technique of dual pairing vector spaces and demonstrate that the scheme is secure under the hardness of the n-extended decisional Diffie–Hellman assumption. Therefore, the proposed scheme is secure and can be applied in scenarios that require hierarchical authorization. To the best of the authors’ knowledge, no PEKS variant schemes with this property have been previously designed.