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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Subliminal channel ： ウィキペディア英語版 Subliminal channel 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 dilemma 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. 〕 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 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. == 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 broadband〔Gustavus 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 channels 〔Gustavus 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.〕 At signing the parameter $k$ has to be set random. For the broadband channel this parameter is instead set with a subliminal message $m\text{'}$. # Key generation ## choose prime $p\; =\; 2347$ ## choose prime $q\; =\; 23$ ## calculate generator $g\; =\; 266$ ## choose authentication key $x\; =\; 1468$ and send it securely to the receiver ## calculate public key $y\; =\; g^x$ mod $p\; =\; 2100$ # Signing ## choose message $m\; =\; 1337$ ## (hash function $H(m)$ is here substituted with a modulo reduction by 107) calculate message hash value $h\; =\; m$ mod $q\; =\; 1337$ mod $107\; =\; 53$ ## instead of random value $k\; =\; ?$ subliminal message $m\text{'}\; =\; 17$ is chosen ## calculate inverse of the subliminal message $m\text{'}^\; =\; 19$ mod $23$ ## calculate signature value $r\; =\; (g^k$ mod $p)$ mod $q\; =\; (266^$ mod $2347)$ mod $23\; =\; 12$ ## calculate signature value $s\; =\; k^$ * (h + x *r) mod $q\; =\; 19$ * (53 + 1468 * 12) mod $23\; =\; 3$ ## sending message with signature triple $(1337;\; 12,\; 3)$ # Verifying ## receiver gets message triple $(m;\; r,\; s)\; =\; (1337;\; 12,\; 3)$ ## calculate message hash $h\; =\; H(m)$ mod $q\; =\; 1337$ mod $107\; =\; 53$ ## calculate inverse $w\; =\; s^$ mod $q\; =\; 8$ ## calculate $u\_1\; =\; (h$ * w) mod $q\; =\; 53$ * 8 mod $23\; =\; 10$ ## calculate $u\_2\; =\; (r$ * w) mod $q\; =\; 12$ * 8 mod $23\; =\; 4$ ## calculate signature $v\; =\; (g^$ * y^ mod $p)$ mod $q\; =\; (266^$ * 2100^4 mod $2347)$ mod $23\; =\; 12$ ## since $v\; =\; r$, the is signature valid # Message extraction on receiver side ## from triple (1337; 12, 3) ## extract message $m\text{'}\; =\; 8$ * (53 + 1468 * 12) mod $23\; =\; 17$ The formula for message extraction is derived by transposing the signature value $s$ calculation formula. * $s\; =\; m\text{'}^$ * (h + xr) mod $q$ * $s$ * m' = h + xr mod $q$ * $m\text{'}\; =\; s^$ * (h + xr) mod $q$ 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Subliminal channel」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. 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