Fair Blind Signatures

A blind signature scheme is a protocol for obtaining a sig- nature from a signer such that the singer’s view of the protocol can- not be linked to the resulting message-signature pair. Blind signature schemes are used in anonymous digital payment systems. Since the ex- isting proposals of blind signature schemes provide perfect unlinkability, such payment systems could be misused by criminals, e.g. to safely ob tain a ransom or to launder money. In this paper, a new type of blind signature schemes called fair blind signature schemes is proposed. Such schemes have the additional property that a trusted entity can deliver information allowing the signer to link his view of the protocol and the message-signature pair. Two types of fair blind signature schemes are distinguished and several realizations are presented.