The invention relates to a simple gesture control system that permits accurate, simple, and secure input of personal identification numbers (PIN) by visually impaired or blind persons at a payment terminal or automatic teller machine. By means of the user interface, invisible keys can be selected by means of simple wiping gestures. Here, any necessity for visual or spoken feedback or localization of operating elements is rendered unnecessary. As a result, blind or visually impaired users can input a secret number by means of touch operation automatically and without outside help. In this way, account is also taken of the statutory requirements in different countries. According to one embodiment of the invention, the selection of the invisible key is separated from the actuation thereof in that, in order to actuate the selected invisible key, an individual touch input, here a typing gesture, is required. This has the advantage that the user can explore the user interface first before he carries out an actual symbol input. The method can be implemented on conventional, already existing hardware. The user interface has been conceived in accordance with reliable and user-centric guidelines.