Security Issues of ID-Based on/offline Signcryption Revisited

ID-based offline and online signcryption (OOSE) consists of two phases; offline signcryption (OffSE) and online signcryption (OnSE). Through OffSE phase, senders can pretreat heavy operations, which enables for senders to perform relatively light operations in OnSE phase. In the paper, we raise an access issue between two phases; if a sender A can access OffSE then is it right to permit A to naturally get an access to OnSE? In practice, however, to connect two phases, there must be secure long-term memory spaces in which OffSE secrets are stored. If those memories are corrupted by accident, then anyone can freely proceed with the next OnSE phase. What we claim here is that, for better security, each phase must not affect the security of the other phase under the corruption of each phase. We firstly present these offline and online attacks in OOSE and analyzed them through relevant OOSE results.