Classification of Signature-only Signature Models.

We introduce a set of criterions for classifying signature-only signature models. By the criterions, we classify signature models into 5 basic types and 69 general classes. Theoretically, 21140 kinds of signature models can be deduced by appropriately combining different general classes. The result comprises almost existing signature models. We also contribute a lot of new signature models. Moreover, we find the three signature models, i.e., group-nominee signature, multi-nominee signature and threshold-nominee signature, are of great importance in light of our classification.