Improved Identity-Based Online/Offline Encryption

The notion of online/offline encryption was put forth by Guo, Mu and Chen (FC 2008), where they proposed an identity-based scheme called identity-based online/offline encryption (IBOOE). An online/offline encryption separates an encryption into two stages: offline and online. The offline phase carries much more computational load than the online phase, where the offline phase does not require the information of the message to be encrypted and the identity of the receiver. Subsequently, many applications of IBOOE have been proposed in the literature. As an example, Hobenberger and Waters (PKC 2014) have recently applied it to attribute-based encryption. In this paper, we move one step further and explore a much more efficient variant. We propose an efficient semi-generic transformation to obtain an online/offline encryption from a tradition identity-based encryption (IBE). Our transformation provides a new method to separate the computation of receiver's identity into offline and online phases. The IBOOE schemes using our transformation saves one group element in both offline and online phases compared to other IBOOE schemes in identity computing. The transformed scheme still maintains the same level of security as in the original IBE scheme.