  Physical isolation, so called air-gapping, is an effective method for
protecting security-critical computers and networks. While it might be possible
to introduce malicious code through the supply chain, insider attacks, or
social engineering, communicating with the outside world is prevented.
Different approaches to breach this essential line of defense have been
developed based on electromagnetic, acoustic, and optical communication
channels. However, all of these approaches are limited in either data rate or
distance, and frequently offer only exfiltration of data. We present a novel
approach to infiltrate data to and exfiltrate data from air-gapped systems
without any additional hardware on-site. By aiming lasers at already built-in
LEDs and recording their response, we are the first to enable a long-distance
(25m), bidirectional, and fast (18.2kbps in & 100kbps out) covert communication
channel. The approach can be used against any office device that operates LEDs
at the CPU's GPIO interface.
