  Smartwatches enable many novel applications and are fast gaining popularity.
However, the presence of a diverse set of on-board sensors provides an
additional attack surface to malicious software and services on these devices.
In this paper, we investigate the feasibility of key press inference attacks on
handheld numeric touchpads by using smartwatch motion sensors as a
side-channel. We consider different typing scenarios, and propose multiple
attack approaches to exploit the characteristics of the observed wrist
movements for inferring individual key presses. Experimental evaluation using
commercial off-the-shelf smartwatches and smartphones show that key press
inference using smartwatch motion sensors is not only fairly accurate, but also
comparable with similar attacks using smartphone motion sensors. Additionally,
hand movements captured by a combination of both smartwatch and smartphone
motion sensors yields better inference accuracy than either device considered
individually.
