Identity based hybrid signcryption revisited

Signcryption is a cryptographic primitive which combines both the functions of digital signature and public key encryption logically in single step, and with a computational cost significantly less than required by the traditional signature-then-encryption approach. Identity based cryptosystem is a public key cryptosystem in which public key can be any arbitrary string. Hybrid cryptosystem combines the convenience of a public key cryptosystem with the efficiency of a symmetric cryptosystem. Dent [4] has given security models for hybrid signcryption scheme with insider security. He has given two security models for hybrid signcryption KEM: key indistinguishability and message indistinguishability model. Hybrid signcryption in identity base setting was given by Fagen Li et al. [10]. In this paper [10] only one security model key indistinguishability is considered. Our contribution in this paper is three fold: First we give new proof for IDB hybrid signcryption in Dent [4] security model. Second for the confidentiality of hybrid signcryption, we improve the bound as compared to Dent [4]. Third we also give the example of identity based hybrid signcryption based on [11].