Public-key encryption with bidirectional keyword search and its application to encrypted emails

Abstract

A traditional Public-key Encryption scheme with Keyword Search (PEKS) allows multiple senders to encrypt keywords under the public key of a receiver such that the receiver can search on these encrypted keywords using his/her searching secret key. In encrypted email systems, an email user not only needs to search on encrypted emails received from other users, but also needs to search on encrypted emails sent to other users. Motivated by this, the paper proposes a cryptographic method to allow these two types of user (i.e., senders and receivers) to search on encrypted keywords, which is called Public-key Encryption with Bidirectional Keyword Search (PEBKS). We give formal definitions of a PEBKS scheme and its indistinguishable security model to capture the scenario that no adversary can efficiently distinguish two ciphertexts of keywords from each other, even if the adversary can adaptively obtain search trapdoors of many keywords. Specifically, we propose a concrete PEBKS scheme, whose security relies on a standard hard problem, i.e., bilinear Diffie–Hellman problem, in the random oracle model. Finally, we simulate the proposed PEBKS scheme to assess its practicability and convinces that its feasibility to be applied to encrypted email systems.