Multi-user Public Key Timed-Release Searchable Encryption

Public key timed-release searchable encryption (PKTRSE) can be used to solve the time-dependent cipher text retrieval problem. In one to one PKTRSE, the sender transmits the encrypted message to the server and wants it to be searched and decrypted by the unique appointed receiver after the release time. When such PKTRSE is applied to encrypt a message for multiple recipients with the same release time, its cipher text size depends on the user scale. To achieve PKTRSE with constant costs from the encryptor´s and decryptor´s point of view, by borrowing the technique of identity-based broad-cast encryption, we propose a cryptosystem of one to many PKTRSE which we call multi-user PKTRSE (MUPKTRSE). In our MUPKTRSE model, the sender transmits an encrypted message so that only the intended authorized user group member can search the target cipher text containing specified keywords before a pre-set release time, but each authorized receiver cannot decrypt it until the release time in the future. In this paper, we begin by explaining what is MUPKTRSE and introducing the application scenario of MUPKTRSE. Then, we formalize the notion of MUPKTRSE and its security game model. Finally, we give a concrete scheme which is secure under the q-DBDHI assumption without random oracles.