  With the emerging of touch-less human-computer interaction techniques and
gadgets, mid-air hand gestures have been widely used for authentication. Much
literature examined either the usability or security of a handful of gestures.
This paper aims at quantifying usability and security of gestures as well as
understanding their relationship across multiple gestures. To study
gesture-based authentication, we design an authentication method that combines
Dynamic Time Warping (DTW) and Support Vector Machine (SVM), and conducted a
user study with 42 participants over a period of 6 weeks. We objectively
quantify the usability of a gesture by the number of corners and the frame
length of all gesture samples, quantify the security using the equal error rate
(EER), and the consistency by EER over a period of time. Meanwhile, we obtain
subjective evaluation of usability and security by conducting a survey. By
examining the responses, we found that the subjective evaluation confirms with
the objective ones, and usability is in inverse relationship with security. We
studied the consistency of gestures and found that most participants forgot
gestures to some degree and reinforcing the memorization of gestures is
necessary to improve the authentication performance. Finally, we performed a
study with another 17 participants on shoulder surfing attacks, where attackers
can observe the victims multiple times. The results show that shoulder surfing
does not help to boost the attacks.
