Self-escrowed cash against user blackmailing

Protecting customer privacy is an important requirement when designing electronic cash systems. However, there is also concern that anonymous cash systems can be misused for criminal activities. Particularly blackmailing is in fact more severe in digital cash systems than in paper-based systems. This is because on the one hand the blackmailer is able to avoid physical contact and on the other hand there are no recognizable note numbers. To prevent such activities, several cash systems have been proposed where one or a collection of trustees can revoke the anonymity of a user. However, this also introduces a serious risk that this revocation ability is misused. In this paper we show that the problem of user blackmailing can be solved without this risk. In our proposal, instead of a trustee, it is rather the blackmailed person who reveals the required information to trace extorted coins without compromising any of her secrets. We show how to derive such systems from concrete existing proposals for anonymity-revocable cash systems with passive trustee.