The Prisoners’ Problem and the Subliminal Channel
Two accomplices in a crime have been arrested and are about to be locked in widely separated cells. Their only means of communication after they are locked up will he by way of messages conveyed for them by trustees -- who are known to be agents of the warden. The warden is willing to allow the prisoners to exchange messages in the hope that he can deceive at least one of them into accepting as a genuine communication from the other either a fraudulent message created by the warden himself or else a modification by him of a genuine message. However, since he has every reason to suspect that the prisoners want to coordinate an escape plan, the warden will only permit the exchanges to occur if the information contained in the messages is completely open to him -- and presumably innocuous. The prisoners, on the other hand, are willing to accept these conditions, i.e., to accept some risk of deception in order to be able to communicate at all, since they need to coordinate their plans. To do this they will have to deceive the warden by finding a way of communicating secretly in the exchanges, i.e., of establishing a “subliminal channel” between them in full view of the warden, even though the messages themselves contain no secret (to the warden) information‡. Since they anticipate that the warden will try to deceive them by introducing fraudulent messages, they will only exchange messages if they are permitted to authenticate them.
Unable to display preview. Download preview PDF.
- 1.G. J. Simmons, “Message Authentication Without Secrecy,” in Secure Communications and Asymmetric Cryptosystems, (ed. by G. J. Simmons) AAAS Selected Symposia Series, Westview Press, Boulder, CO (1982), pp. 105–139.Google Scholar
- 2.G. J. Simmons, “Verification of Treaty Compliance - Revisited,” Proceedings of the 1983 IEEE Symposium on Security and Privacy, Oakland, CA (April 25–27, 1983), to appear.Google Scholar
- 4.H. Ong and C. P. Schnorr, “Signatures through Approximate Representations by Quadratic Forms,” Proceedings of the IEEE Workshop on Communications Security (CRYPTO’83), University of California, Santa Barbara, CA (August 26–30, 1983), to appear.Google Scholar
- 6.D. H. Lehmer, “Computer Technology Applied to the Theory of Numbers in Studies in Number Theory edited by W. J. LeVeque, M.A.A. Studies in Mathematics, Vol. 6, Prentice Hall (1969), 132–133.Google Scholar
- 7.G. J. Simmons and D. H. Holdridge, “Forward Search as a Cryptanalytic Tool,” Proceedings of the 1982 Symposium on Security and Privacy, Oakland, CA (April 26–28, 1982 ), 117–128.Google Scholar