[1] 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.
A narrow-band channel cannot transport maximal information, but it can be used to send the authentication key or datastream.
The cure for this was found by cryptologists at the Centrum Wiskunde & Informatica in Amsterdam, who developed a Zero-knowledge proof that n is of the form n = pq.
The second is the base64 representation of the public key - again, supposed to be all random gibberish, but the English-readable message "//This+is+Christopher+Drakes+PGP+public+key//Who/What+is+watcHIng+you//" has been inserted.
A modification to the Brickell and DeLaurentis signature scheme provides a broadband channel without the necessity to share the authentication key.
[6] With the help of the zero-knowledge proof and the commitment scheme it is possible to prevent the usage of the subliminal channel.