Fair Blind T hreshold Signatures in Wallet w ith Observers

Abstract

In this paper, we propose efficient fair blind (t, n) threshold signature schemes in wallet with observers. By these schemes, any t out of n signers in a group can represent the group to sign fair blind threshold signatures, which can be used in anonymous e-cash systems. Since blind signature schemes provide perfect unlinkability, such e-cash systems can be misused by criminals, e.g. to safely obtain a ransom or to launder money. Our schemes allow the judge (or the judges) to deliver information allowing anyone of the t signers to link his view of the protocol and the message-signature pair.