Signing with Multiple ID's and a Single Key

                                          Miroslaw Kutylowski
               Wroclaw University of Technology, Wroclaw, Poland
                                               Jun Shao
                 Zhejiang Gongshang University, Hangzhou, P.R.C. 



We propose a solution for  electronic identity cards that makes it possible to create multiple
identities for a person creating digital signatures with just one private key stored on his 
personal electronic identity card. The public keys corresponding to this secret key belong 
to different sectors, devoted to different applications or activity areas (like private use and  
signing in behalf of a company or an authority).

We provide strong privacy guarantees called unlinkability. It means that given two public keys
from different sectors and some signatures corresponding to these keys, it is unfeasible to say if
these keys (and signatures) come from the same person. The proof is performed in the random 
oracle model and reduces linkability to the Decisional Diffie-Hellman Problem.

Our proposal extends the idea of Restricted Identification introduced in German personal identity 
cards from unlinkable sector identification to unlinkable sector signatures.


research supported by Polish Ministry of Research and Higher Education,
grant O R00 0015 07,  Foundation for Polish Science, ``MISTRZ'' Programme, and 
National Natural Science Foundation of China, grant  61003308