A Survey and Taxonomy Aimed at the Detection and Measurement of Covert Channels

New viewpoints of covert channels are presented in this work. First, the origin of covert channels is traced back to acc ess control and a new class of covert channel, air-gap covert channels, is presented. Second, we study the design of covert channels and provide novel insights that differentiate the research area of undetectable communication from that of covert channels. Third, we argue that secure systems can be characterized as fixed-source systems or continuous-source systems, i.e., systems whose security is compromised if their design allows a covert channel to communicate a small, fixed amount of information or communicate information at a sufficiently high, continuous rate, respectively. Consequently, we challenge the traditional method for measuring covert channels, which is based on Shannon capacity, and propose that a new measure, steganographic capacity, be used to accurately assess the risk posed by covert channels, particularly those affecting fixed-source systems. Additionally, our comprehensive review of covert channels has led us to the conclusion that important properties of covert channels have not been captured in previous taxonomies. We, therefore, present novel extensions to existing taxonomies to more accurately characterize covert channels.