In 1978 the United States was considering adopting a national security protocol designed to enable the USSR to verify how many Minuteman missiles the United States had emplaced in a field of 1000 silos without revealing which silos actually contained missiles. For this protocol to have been acceptable to the USSR, the messages would have had to be digitally signed with signatures which the USSR could verify were authentic, but which the United States could not forge. Subliminal channels were the discovery that these digital signatures could host undetectable covert channels. In general, any time redundant information is introduced into a communication to provide an overt function such as digital signatures, error detection and/or correction, authentication, etc. it may be possible to subvert the purported function to create a covert (subliminal) communications channel. This paper recounts the development of subliminal channels from their origins when only a couple of bits could be communicated covertly to today when potentially a couple of hundred bits can be concealed in signatures generated using the most popular digital signature schemes.
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