  We address the problem of shoulder-surfing attacks on authentication schemes
by proposing IllusionPIN (IPIN), a PIN-based authentication method that
operates on touchscreen devices. IPIN uses the technique of hybrid images to
blend two keypads with different digit orderings in such a way, that the user
who is close to the device is seeing one keypad to enter her PIN, while the
attacker who is looking at the device from a bigger distance is seeing only the
other keypad. The user's keypad is shuffled in every authentication attempt
since the attacker may memorize the spatial arrangement of the pressed digits.
  To reason about the security of IllusionPIN, we developed an algorithm which
is based on human visual perception and estimates the minimum distance from
which an observer is unable to interpret the keypad of the user. We tested our
estimations with 84 simulated shoulder-surfing attacks from 21 different
people. None of the attacks was successful against our estimations. In
addition, we estimated the minimum distance from which a camera is unable to
capture the visual information from the keypad of the user. Based on our
analysis, it seems practically almost impossible for a surveillance camera to
capture the PIN of a smartphone user when IPIN is in use.
