Enhancing the security of perfect blind DL-signatures

We enhance the security of Schnorr blind signatures against the novel one-more-forgery of Schnorr [Sc01] and Wagner [W02] which is possible even if the discrete logarithm is hard to compute. We show two limitations of this attack. Firstly, replacing the group G by the s-fold direct product G×s increases the work of the attack, for a given number of signer interactions, to the s-power while increasing the work of the blind signature protocol merely by a factor s. Secondly, we bound the number of additional signatures per signer interaction that can be forged effectively. That fraction of the additional forged signatures can be made