Digital payment systems enabling security and unobservability

In present-day cashless payment systems, the banks and (by installing a Trojan Horse) even the manufacturers of the computer equipment used could easily observe who pays what amount to whom and when. With the increasing digitization of these systems, e.g. point-of-sale terminals and home banking, the amount of transaction data and their computerization drastically increases. Therefore these payment systems become completely unacceptable, since compiling dossiers on the lifestyle and whereabouts of all clients will become easy. We describe the digital payment systems enabling unobservability of clients and arrange them in a general model to compare their different degrees of unobservability and their different levels of security. Since no single system has all desired features, we propose a suitable synthesis.