In cryptography, subliminal channels are covert channels that can be used to communicate secretly in normal looking communication over an insecure channel.Gustavus J. Simmons. The Prisoners Problem and the Subliminal Channel. In Advances in Cryptology – CRYPTO ’83, pages 51–67, New York, 1984. Lecture Notes in Computer Science, ed. D. Chaum. 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.Gustavus J. Simmons. The subliminal channel and digital signatures. In Proc. of the EUROCRYPT 84 workshop on Advances in Cryptology – theory and application of cryptographic techniques, pages 364–378, New York, NY, USA, 1985. Springer-Verlag New York, Inc.
Examples
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 broadbandGustavus J. Simmons. Subliminal communication is easy using the DSA. In EUROCRYPT ’93: Workshop on the theory and application of cryptographic techniques on Advances in cryptology, pages 218–232, Secaucus, NJ, USA, 1994. Springer-Verlag New York, Inc. and three subliminal narrow-band channelsGustavus J. Simmons. The subliminal channel in the U.S. Digital Signature Algorithm (DSA), in Proceedings of the 3rd Symposium on State and Progress of Research in Cryptography (SPRC '93), Rome, Italy, February 15–16, 1993.
Improvements A modification to the Brickell and DeLaurentis signature scheme provides a broadband channel without the necessity to share the authentication key.Gustavus J. Simmons. A Secure Subliminal Channel (?). In CRYPTO ’85: Advances in Cryptology, pages 33–41, London, UK, 1986. Springer-Verlag. The Newton channel is not a subliminal channel, but it can be viewed as an enhancement.Ross J. Anderson, Serge Vaudenay, Bart Preneel, and Kaisa Nyberg. The Newton Channel. In Proceedings of the First International Workshop on Information Hiding, pages 151–156, London, UK, 1996. Springer-Verlag.
Countermeasures With the help of the zero-knowledge proof and the commitment scheme it is possible to prevent the usage of the subliminal channel.Yvo Desmedt. Abuses in Cryptography and How to Fight Them. In CRYPTO ’88: Proceedings of the 8th Annual International Cryptology Conference on Advances in Cryptology, pages 375–389, London, UK, 1990. Springer-Verlag.Yvo Desmedt. "Subliminal-free authentication and signature". p. 24 of Christoph G. Günther, editor. "Advances in Cryptology - EUROCRYPT '88". 1988.
This countermeasure has a 1-bit subliminal channel because for is the problem that a proof can succeed or purposely fail.
Another countermeasure can detect, and not prevent, the subliminal usage of the randomness.
Notes
References
Bruce Schneier. Applied Cryptography, Second Edition: Protocols, Algorithms, and Source Code in C, 2. Ed. Wiley Computer Publishing, John Wiley & Sons, Inc., 1995.
External links