There are significant technical challenges in developing and
deploying technologies for user identification, particularly
on touchscreens. Foremost, and perhaps most challenging,
is that the best techniques avoid instrumenting the user.
Further, there should be minimal or no instrumentation of
the environment, as external infrastructure is costly and
prohibits mobility. Furthermore, the technique must be fast
and robust, identifying a large number of users both sequentially
and simultaneously. Additionally, it should be
inexpensive, easily deployable, sufficiently compact, and
have low power requirements, thus making integration into
mobile devices feasible. Currently, we are not aware of any
system that satisfies all of these requirements.