On Characterizing and Measuring Out-of-Band Covert Channels
Brent Carrara, Carlisle M. Adams · 2015
A methodology for characterizing and measuring out-of-band covert channels (OOB-CCs) is proposed and used to evaluate covert-acoustic channels (i.e., covert channels established using speakers and microphones). OOB-CCs are low-probability of detection/low-probability of interception channels established using commodity devices that are not traditionally used for communication (e.g., speaker and microphone, display and FM radio, etc.). To date, OOB-CCs have been declared "covert" if the signals used to establish these channels could not be perceived by a human adversary. This work examines OOB-CCs from the perspective of a passive adversary and argues that a different methodology is required in order to effectively assess OOB-CCs. Traditional communication systems are measured by their capacity and bit error rate; while important parameters, they do not capture the key measures of OOB-CCs: namely, the probability of an adversary detecting the channel and the amount of data that two covertly communicating parties can exchange without being detected. As a result, the adoption of the measure steganographic capacity is proposed and used to measure the amount of data (in bits) that can be transferred through an OOB-CC before a passive adversary's probability of detecting the channel reaches a given threshold. The theoretical steganographic capacity for discrete memoryless channels as well as additive white Gaussian noise channels is calculated in this paper and a case study is performed to measure the steganographic capacity of OOB covert-acoustic channels, when a passive adversary uses an energy detector to detect the covert communication. The case study reveals the conditions under which the covertly communicating parties can achieve perfect steganography (i.e., conditions under which data can be communicated without risk of detection).