Ein mißbrauchfreies anonymes elektronisches Zahlungssystem

While anonymous payments are necessary for privacy reasons, they can as well be misused for criminal activities: Anonymous electronic cash can be used to conduct crimes that would be impossible with physical cash. As potential misuse of anonymity cannot be tolerated, the allowed anonymity has to be restricted. Payment systems with restricted anonymity are commonly based on the concept of a trusted third party that is always able to subsequently revoke the anonymity of any payment. If anonymity was misused, it is always possible to investigate the delinquent. However, both the customers as well as law enforcement have to rely on the trusted third party to revoke the anonymity of a payment if and only if a judical warrant was issued. Thus, the trusted third party must be subject to rigorous restrictions and controls. Creating an instance that is able to control the trusted third party is still an open problem. Therefore, in our opinion, the trusted third party approach is not suitable for anonymous payment systems. In this dissertation we introduce FlexiCash, a new anonymous payment system that uses a novel approach to prevent misuse of anonymity independent of trusted third parties. Although FlexiCash is well comparable to physical cash, it does not only offer stronger anonymity but has also superior deanonymization mechanisms. This is possible by integrating an audit mechanism in FlexiCash that cannot be found in any other payment system: At regular intervals previous payments can be audited by the payer himself. Then the payer can check, whether the anonymity of his payments has been revoked. In this case, a judical warrant must be available otherwise the deanonymization was provably illegal and can be prosecuted. If no deanoymization is detected by the payer, it is guaranteed that the payments have been and will remain anonymous. No other payment system with restricted anonymity can guarantee such strong anonymity.