http://www.informatik.tu-darmstadt.de/GK/participants/kuegler/pkc01_marking....: Marking: A Privacy Protecting Approach Against Blackmailing Dennis Kuegler and Holger Vogt Blind signature based anonymous payment systems [Cha83] have been invented for privacy protecting payments over the internet. However, it was discovered by von Solms and Naccache [vSN92] that unconditional anonymity may be misused by criminals: A blackmailer can exploit the properties of the used blind signature to receive blackmailed money from his victim so that neither the victim nor the bank are able to recognize the blackmailed coins later. Furthermore, the blackmailed coins can be transferred anonymously via an unobservable broadcast channel (e.g. a newsgroup). This attack is called the perfect crime, as it is impossible to identify or trace the blackmailer. To solve anonymity related problems as blackmailing, money laundering, or illegal purchases, payment systems with revokable anonymity have been proposed [CMS96, JY96, FTY96, JY97]. In these payment systems trusted third parties are able to revoke the anonymity of the users at any time. In our opinion blackmailing is the most serious drawback of the known payment systems offering unconditional anonymity. Attacks like money laundering and illegal purchases aren't a major problem in anonymous electronic payment systems, as these problems are even worse with physical cash [Fro96], because in anonymous electronic payment systems the bank always knows how much a customer withdraws and how much a person deposits. Thus the bank is able to detect either the initiator or the recipient of a suspicious transaction. In this paper we will show how to fight blackmailing without restricting the anonymity of users as it is done in systems with revokable anonymity. We present a new online payment scheme, which offers unconditional anonymity, but does not suffer from the blackmailing attack described above.