  We report the first active acoustic side-channel attack. Speakers are used to
emit human inaudible acoustic signals and the echo is recorded via microphones,
turning the acoustic system of a smart phone into a sonar system. The echo
signal can be used to profile user interaction with the device. For example, a
victim's finger movements can be inferred to steal Android phone unlock
patterns. In our empirical study, the number of candidate unlock patterns that
an attacker must try to authenticate herself to a Samsung S4 Android phone can
be reduced by up to 70% using this novel acoustic side-channel. Our approach
can be easily applied to other application scenarios and device types. Overall,
our work highlights a new family of security threats.
