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In cryptography, subliminal channels are covert channels that can be used to communicate secretly in normal looking communication over an insecure channel.[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.[2][a]
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.
- Subliminal channels can be classified into broadband and narrowband channel types.
- Broadband and narrowband channels can exist in the same datastream.
- The broadband channel uses almost all available bits that are available to use. This is commonly understood to mean {≥50% but ≤90%} channel utilization.
- Every channel which uses fewer bits is called a narrow-band channel.
- The additional used bits are needed for further protection, e.g., impersonation.
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.
- ^ a b 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.
- ^ 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. doi:10.1007/3-540-39757-4_25
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