Shoulder surfing is a term that implies the theft of sensitive information when interacting with digital devices. It is of particular concern when authenticating in public places. We present a passive method to prevent it based on the use of an automultiscopic display, originally designed to generate three-dimensional effects, to project different images in each direction, producing a blurry image unrecog-nizable to any shoulder surfer; while the genuine user can observe the display without penalty. We demonstrate that it is possible to achieve privacy criteria using two digital displays operating at 133 Hz by choosing the blocked views as prudently filtered versions of the original pattern. In addition, we propose a merit function that allows to automatically evaluate the readability of each displayed view.