Modern, off-the-shelf smartphones provide a rich set of possible touchscreen interactions, but knowledge-based authentication schemes still rely on simple digit or character input. Previous studies examined the shortcomings of such schemes based on unlock patterns, PINs, and passcodes.

In this paper, we propose to integrate pressure-sensitive touchscreen interactions into knowledge-based authentication schemes. By adding a (practically) invisible, pressure-sensitive component, users can select stronger PINs that are harder to observe for a shoulder surfer. We conducted a within-subjects design lab study (n = 50) to compare our approach termed force-PINs with standard four-digit and six-digit PINs regarding their usability performance and a comprehensive security evaluation. In addition, we conducted a field study that demonstrated lower authentication overhead. Finally, we found that force-PINs let users select higher entropy PINs that are more resilient to shoulder surfing attacks with minimal impact on the usability performance.