Subliminal channels
In cryptography, subliminal channels are covert channels that can be used to communicate secretly in normal looking communication over an insecure channel. Subliminal channels in digital signature crypto systems were found in 1984 by Gustavus Simmons.
Simmons describes how the "Prisoners' Problem" can be solved through parameter substitution in digital signature algorithms. (Note that Simmons' Prisoners' Problem is not the same as the Prisoner's Dilemma.)
Signature algorithms like ElGamal and DSA have parameters which must be set with random information. He shows how one can make use of these parameters to send a message subliminally. Because the algorithm's signature creation procedure is unchanged, the signature remains verifiable and indistinguishable from a normal signature. Therefore, it is hard to detect if the subliminal channel is used.
The broadband and the narrow-band channels can use different algorithm parameters. A narrow-band channel cannot transport maximal information, but it can be used to send the authentication key or datastream.
Research is ongoing : further developments can enhance the subliminal channel, e.g., allow for establishing a broadband channel without the need to agree on an authentication key in advance. Other developments try to avoid the entire subliminal channel.
An easy example of a narrowband subliminal channel for normal human-language text would be to define that an even word count in a sentence is associated with the bit "0" and an odd word count with the bit "1". The question "Hello, how do you do?" would therefore send the subliminal message "1".
The Digital Signature Algorithm has one subliminal broadband and three subliminal narrow-band channels
At signing the parameter has to be set random. For the broadband channel this parameter is instead set with a subliminal message .
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