Unformatted text preview: as much trouble impersonating Alice as would Walter or anyone else. DSA
There is also a subliminal channel in DSA (see Section 20.1) [1468,1469,1473]. In fact, there are several. The simplest subliminal channel involves the choice of k. It is supposed to be a 160bit random number. However, if Alice chooses a particular k, then Bob, who also knows Alice’s private key, can recover it. Alice can send Bob a 160bit subliminal message in each DSA signature; everyone else simply verifies Alice’s signature. Another complication: Since k should be random, Alice and Bob need to share a onetime pad and encrypt the subliminal message with the onetime pad to generate a k. DSA has subliminal channels that do not require Bob to know Alice’s private key. These also involve choosing particular values of k, but cannot be used to send 160 bits of information. This scheme, presented in [1468,1469], allows Alice and Bob to exchange one bit of subliminal information per signed message. (1) Alice and Bob agree on a random prime, P (different from the parameter p in the signature scheme). This is their secret key for the subliminal channel. (2) Alice signs an innocuous message, M. If she wants to send Bob the subliminal bit, 1, she makes sure the r parameter of the signature is a quadratic residue modulo P. If she wants to send him a 0, she makes sure the r parameter is a quadratic nonresidue modulo P. She does this by signing the message with random k values until she gets a signature with an r with the requisite property. Since quadratic residues and quadratic nonresidues are equally likely, this shouldn’t be too difficult. (3) Alice sends the signed message to Bob. (4) Bob verifies the signature to make sure the message is authentic. Then he checks whether r is a quadratic residue or a quadratic nonresidue modulo P and recovers the subliminal bit. Previous Table of Contents Next Products  Contact Us  About Us  Privacy  Ad Info  Home Use of this site is subject to certain Terms & Conditions, Copyright © 19962000 EarthWeb Inc....
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 Fall '10
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 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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