Degree Name

Master of Computer Science - Research


School of Computer Science and Software Engineering


The sign-and-then-encrypt approaches have been widely adopted in secure applications. The computational cost of these approaches is the sum of the corresponding signing cost and encryption cost. Signcryption achieves the same e®ect as sign-and-then-encrypt, but is more e±cient, since the computational cost is less than sign-and-then-encrypt. In this thesis, we present further studies on signcryption to explore some nice properties for better applicability.

We notice that the features of identity-based cryptography have not been well in- vestigated in signcryption. We also observe that there is a potential to further improve computational e±ciency of signcryption by pre-computation. We therefore propose identity-based online/o²ine signcryption to capture those features. We provide the no- tion of identity-based online/o²ine signcryption and the corresponding security model. We then present two e±cient constructions of identity-based online/o²ine signcryp- tion. Besides the computational e±ciency due to signcryption, pre-computation in the online/o²ine approach further reduces the computational overhead. We show that our schemes are secure against chosen ciphertext attacks and existential unforgeable against chosen message attacks respectively in the random oracle model.

As an extension to our study, a generic construction of identity-based online/o²ine signcryption and a generic online/o²ine broadcast signcryption scheme will be also presented. In our construction, any identity-based signature and encryption schemes can be applied to our online/o²ine signcryption in identity-based settings. A security proof is presented to show the invulnerability of our generic schemes.