Figure 2 gives an overview over existing systems. Extortion not prev. Our system Extortion off line Extortion not prev. All schemes require the participation of the trustee in the opening of an account or even in the withdrawal of coins. However, they 1 The author s work was granted by a postdoctoral fellowship of the NATO Scientific Committee disseminated by the DAAD. are unable to prevent extortion attacks and the use of blindfolding protocols.
Different coins spent by the same user are unlinkable.
It is either realized by a separate mechanism or in the same way as user tracing. The goal of this paper is to present an efficient and secure anonymous digital payment systems satisfying this requirement. A comparison with their scheme is given in Section 8. The outline of the paper is as follows. In Sections 2 we give a brief overview of digital payment systems.