  It has been assumed that the physical separation (air-gap) of computers
provides a reliable level of security, such that should two adjacent computers
become compromised, the covert exchange of data between them would be
impossible. In this paper, we demonstrate BitWhisper, a method of bridging the
air-gap between adjacent compromised computers by using their heat emissions
and built-in thermal sensors to create a covert communication channel. Our
method is unique in two respects: it supports bidirectional communication, and
it requires no additional dedicated peripheral hardware. We provide
experimental results based on implementation of BitWhisper prototype, and
examine the channel properties and limitations. Our experiments included
different layouts, with computers positioned at varying distances from one
another, and several sensor types and CPU configurations (e.g., Virtual
Machines). We also discuss signal modulation and communication protocols,
showing how BitWhisper can be used for the exchange of data between two
computers in a close proximity (at distance of 0-40cm) at an effective rate of
1-8 bits per hour, a rate which makes it possible to infiltrate brief commands
and exfiltrate small amount of data (e.g., passwords) over the covert channel.
