Considering mobile payments, this work explores the user interaction possibilities and limitations given by technology opportunities on the one hand, and
security and privacy constraints on the other hand. The work first develops a number of technical and functional requirements and a detailed payment model. The
verification of the user interaction model is then carried out by means of a mobile-payment app for Android phones. This proof-of-concept implementation is able to show how to combine a sufficiently high degree of security with an efficient user interaction and a high degree of usability in the user interface. The app was tested with regard to the integration of single payment steps into the overall organic payment process, as well as a number of security aspects and the so-called secure element. However, a genera l user acceptance test and usability tests remain.