Anonymity Control in E-Cash Systems

Abstrac t . Electronic cash, and other cryptographic payment systems, offer a level of user anonymity during a purchase, in order to emulate electronically the properties of physical cash exchange. However, it has been noted that there are crime-prevention situations where anonymity of notes is undesirable; in addition there may be regulatory and legal constraints limiting anonymous transfer of funds. Thus pure anonymity of users may be, in certain settings, unacceptable and thus a hurdle to the progress of electronic commerce. The conceptual contribution of this work is based on the claim that given the legal, social, technical and efficiency constraints that are imposed, anonymity should be treated as a Control Parameter facilitating flexibility of the level of privacy of note holders (determined by the dynamic conditions and constraints). In light of this parameterization, we review recently developed technical tools for tracing and anonymity revocation (e.g., owner tracing and coin tracing). We elaborate on the differences in the various technologies with respect to security assumptions and we discuss practical considerations of computational, bandwidth and storage requirements for user, shop, bank and trustees as well as whether the trustees must be on-line or off-line. We also claim that while anonymity revocation can potentially reduce crime it can also produce instances where the severity of the crime is increased as criminals try to social engineer around tracing revocation. To prevent this we suggest the notion of "distress cash." On the technical side, we provide efficiency improvements to a protocol for coin tracing and point at a technical solution for distress cash.