I. INTRODUCTION Today, regardless of the existence of very well developed online banking and payment card infrastructure, no electronic transaction can be made in secret. However, because of many drawbacks of the exchange of regular cash, a way of anonymously transferring cash over the Internet would be very useful. There are at least two ways in which such a system can be built. One of them is to create a virtual currency, an example of which is Bitcoin [17]. Another one is to create an electronic cash system, in which a bank mediates the transfer of cash and the clients exchange anonymous cheques of a specified value. The first method of anonymous exchange of cheques was invented more than 20 years ago by David Chaum, but still it is the basic solution on which many of later protocols are based [1][2][5][6][8][9]. The main subject of this publication is to present a possible attack on this protocol.