Undeniable confirmer signature

In undeniable signature, a signature can only be verified with cooperation of the signer. If the signer refuses to cooperate, it is infeasible to check the validity of a signature. This problem is eliminated in confirmer signature schemes where the verification capacity is given to a confirmer rather than the signer. In this paper, we present a variation of confirmer signature, called undeniable confirmer signature in that both the signer and a confirmer can verify the validity of a signature. The scheme provides a better flexibility for the signer and the user as well as reduces the involvement of designated confirmers, who are usually trusted in practice. Furthermore, we show that our scheme is divertible, i.e., our signature can be blindly issued. This is essential in some applications such as subscription payment system, which is also shown.