Online/offline identity-based signcryption revisited

In this paper, we redefine a cryptographic notion called Online/Offline Identity-Based Signcryption. It is an online/offline version of identity-based signcryption, where most of the computations are carried out offline while the online part does not require any heavy computations such as pairings or multiplications on elliptic curve. It is particularly suitable for power-constrained devices such as smart cards. We give a concrete implementation of online/offline identity-based signcryption, which is very efficient and flexible. Unlike all the previous schemes in the literature, our scheme does not require the knowledge of receiver s information (either public key or identity) in the offline stage. The receiver s identity and the message to be signcrypted are only needed in the online stage. This feature provides a great flexibility to our scheme and makes it practical to use in real-world applications. To our knowledge, our scheme is the first one in the literature to provide this kind of feature. We prove that the proposed scheme meets strong security requirements in the random oracle model, assuming the Strong Diffie-Hellman (SDH) and Bilinear Diffie-Hellman Inversion (BDHI) are computationally hard.