  In recent years, various out-of-band covert channels have been proposed that
demonstrate the feasibility of leaking data out of computers without the need
for network connectivity. The methods proposed have been based on different
type of electromagnetic, acoustic, and thermal emissions. However, optical
channels have largely been considered less covert: because they are visible to
the human eye and hence can be detected, they have received less attention from
researchers. In this paper, we introduce VisiSploit, a new type of optical
covert channel which, unlike other optical methods, is also stealthy. Our
method exploits the limitations of human visual perception in order to
unobtrusively leak data through a standard computer LCD display. Our
experiments show that very low contrast or fast flickering images which are
invisible to human subjects, can be recovered from photos taken by a camera.
Consequentially, we show that malicious code on a compromised computer can
obtain sensitive data (e.g., images, encryption keys, passwords), and project
it onto a computer LCD screen, invisible and unbeknownst to users, allowing an
attacker to reconstruct the data using a photo taken by a nearby (possibly
hidden) camera. In order to demonstrate the feasibility of this type of attack
and evaluate the channel's stealth, we conducted a battery of tests with 40
human subjects. We also examined the channel's boundaries under various
parameters, with different types of encoded objects, at several distances, and
using several kinds of cameras. Our results show that binary data can be leaked
via our covert channel. Further research and discussion may widen the scope of
this field beyond its current boundaries, yielding novel attack paradigms that
exploit the subtle mechanisms of human visual perception.
