User interface for the protected input of characters / symbols, comprising an operating element which has a display area with a touch-sensitive surface, and comprising a first processor being programmed to recognize a touch input which can be executed as a swipe gesture in any area on the touch-sensitive surface of the operating element, wherein the user interface comprises a second processor which is arranged for switching between a plain text mode and a protected input mode and which is further programmed to provide an invisible keyboard on the display area during the protected input mode, wherein the invisible keyboard defines an arrangement of invisible keys, of which exactly one is selected at any time, - wherein to each of the invisible keys a character is assigned which can be entered by means of the respective invisible key, and - wherein no visual or acoustic output of a position or assignment of the selected invisible key is provided during the protected input mode, wherein the first processor is programmed to recognize the touch input, which can be executed as a swipe gesture in the protected input mode, wherein the second processor is programmed to select an invisible key which is arranged in a direction analogous to a direction of the swipe gesture on the invisible keyboard next to the previously selected invisible key, and wherein the first processor is designed to recognize a touch input which can be executed in the protected input mode as a tapping gesture in any area on the touch-sensitive surface of the operating element, wherein the second processor is designed to detect the character which is assigned to the invisible key selected at the time of the tapping gesture, wherein the operating element processes touches with one or more fingertips as touch inputs and transmits them as coordinates or vectors in a coordinate system to the first processor, and wherein the user interface has a security module, in which the first processor is arranged in a first microcontroller with a first communication interface, which connects the operating element to the first processor, and in which the second processor is arranged in a second microcontroller with a second communication interface, which connects the first processor to the second processor, wherein the second processor is designed in the protected input mode for encryption of captured characters. wherein the user interface comprises a second processor which is arranged for switching between a plain text mode and a protected input mode and which is further programmed to provide an invisible keyboard on the display area during the protected input mode, wherein the invisible keyboard defines an arrangement of invisible keys, of which exactly one is selected at any time, - wherein to each of the invisible keys a character is assigned which can be entered by means of the respective invisible key, and - wherein no visual or acoustic output of a position or assignment of the selected invisible key is provided during the protected input mode, wherein the first processor is programmed to recognize the touch input, which can be executed as a swipe gesture in the protected input mode, - wherein to each of the invisible keys a character is assigned which can be entered by means of the respective invisible key, and - wherein no visual or acoustic output of a position or assignment of the selected invisible key is provided during the protected input mode, wherein the first processor is programmed to recognize the touch input, which can be executed as a swipe gesture in the protected input mode, wherein the second processor is programmed to select an invisible key which is arranged in a direction analogous to a direction of the swipe gesture on the invisible keyboard next to the previously selected invisible key, and wherein the first processor is designed to recognize a touch input which can be executed in the protected input mode as a tapping gesture in any area on the touch-sensitive surface of the operating element, wherein the second processor is designed to detect the character which is assigned to the invisible key selected at the time of the tapping gesture, wherein the operating element processes touches with one or more fingertips as touch inputs and transmits them as coordinates or vectors in a coordinate system to the first processor, and wherein the user interface has a security module, in which the first processor is arranged in a first microcontroller with a first communication interface, which connects the operating element to the first processor, and in which the second processor is arranged in a second microcontroller with a second communication interface, which connects the first processor to the second processor, wherein the second processor is designed in the protected input mode for encryption of captured characters.