An Untraceable Electronic Cash System Using Fair Blind Signatures

Since the blind signature scheme has the unlinkability property, it can be used for untraceable electronic cash systems. The unlinkability property protects the privacy of users; however, it may be abused by criminals, e.g., to launder money or to safely get a ransom. The techniques of fair blind signature are developed to withstand the misuse of unlinkability property. In this paper, we propose not only a user-efficient but also a signer-efficient fair blind signature scheme for untraceable electronic cash. Only with the help from a judge or government, the signer or the banker can derive the link between a signature and the instance of the signing protocol which produces the signature when the unlinkability property is abused. Comparing with existing fair blind signature schemes, our method greatly reduces the computational load of an on-line judge. Hence, the proposed method with an efficient judge can let the signer provide more efficient services to the signature requesters. It is also suitable for the limited computation capacities of users such as smart cards or mobile units. So, the proposed scheme is very useful for e-commerce transactions