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Out-of-band covert channels--a survey
Carrara B., Adams C. ACM Computing Surveys49 (2):1-36,2016.Type:Article
Date Reviewed: Sep 30 2016

In recent years, covert channels were shifted back into the focus of research. These channels can be used to communicate in a stealthy way not recognizable by third parties. Typical application scenarios for covert channels are to hide a data exfiltration or to ensure stealthy malware communication. Among the recent domains of covert channel research are network covert channels, local covert channels (especially on smartphones), and the addressed out-of-band channels. Out-of-band covert channels transfer their hidden information using a shared medium, for example air, temperature, or light. For instance, in air, inaudible signals can be used to realize a secret data transfer.

Carrara and Adams present a first survey of out-of-band covert channels. Their work introduces a modified version of Simmons’ Prisoners’ Problem. The work also includes terminology that places out-of-band channels into the context of other types of covert channels. Out-of-band covert channels are then studied per medium, shedding light on channels based on acoustics, light, vibration, magnetics, temperature, and radio frequency. A summary compares these channels. Finally, a novel taxonomy for out-of-band covert channels is presented.

The intended audience, as mentioned by the authors, is the secure systems development community and potential users of covert channels. However, the work serves also as a good starting point for everybody interested in this evolving type of covert channel. The publication is well structured and well written and thus also accessible to people who are new to the field.

However, Carrara’s and Adams’ core idea for a definition of out-of-band covert channels is to split out-of-band covert channels from single-host covert channels (that is, covert channels on a local host). Here lies a weak point of the proposed terminology since so-called “covert physical channels” are referred to as single-host channels by Carrara and Adams although other authors use the term “covert physical channel” also for networked air-gap channels, that is, a type of out-of-band channels. This fact weakens the paper’s definition of its core term. Despite this aspect, the literature survey and analysis of out-of-band channels is a solid work that provides a comprehensive overview and taxonomy on the different types of out-of-band covert channels.

Reviewer:  Steffen Wendzel Review #: CR144798 (1701-0050)
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