E cient Identity-based Signcryption without Random Oracles

In this paper, we propose a new Identity-based signcryption (IBSC) scheme in the standard model. Our scheme shows an improvement of approximately 40% reduction in the ciphertext size when compared to the previously proposed IBSC schemes in the standard model. Further, we argue that the previous IBSC schemes do not provide su cient simulation ability in the security game. We show that with some minor overhead, we are able to correct this. The security reduction of our scheme is based on the hardness of the hashed modi ed decision bilinear Di eHellman problem and the modi ed computational Di e-Hellman problem.