  Mobile device users avoiding observational attacks and coping with
situational impairments may employ techniques for eyes-free mobile unlock
authentication, where a user enters his/her passcode without looking at the
device. This study supplies an initial description of user accu- racy in
performing this authentication behavior with PIN and pattern passcodes, with
varying lengths and visual characteristics. Additionally, we inquire if
tactile-only feedback can provide assistive spatialization, finding that
orientation cues prior to unlocking do not help. Measure- ments of edit
distance and dynamic time warping accuracy were collected, using a
within-group, randomized study of 26 participants. 1,021 passcode entry
gestures were collected and classified, identifying six user strategies for
using the pre-entry tactile feedback, and ten codes for types of events and
errors that occurred during entry. We found that users who focused on orienting
themselves to position the first digit of the passcode using the tactile
feedback performed better in the task. These results could be applied to better
define eyes-free behavior in further research, and to design better and more
secure methods for eyes-free authentication.
