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Unformatted text preview: m or medium without express written permission of EarthWeb is prohibited. Read EarthWeb's privacy statement. To access the contents, click the chapter and section titles. Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth)
Brief Full Advanced Search Search Tips (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book:
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----------- Assuming an eavesdropper can’t get access to the one-time pad used to encrypt the message, this scheme is perfectly secure. A given ciphertext message is equally likely to correspond to any possible plaintext message of equal size. Since every key sequence is equally likely (remember, the key letters are generated randomly), an adversary has no information with which to cryptanalyze the ciphertext. The key sequence could just as likely be: POYYAEAAZX which would decrypt to: SALMONEGGS or BXFGBMTMXM which would decrypt to: GREENFLUID This point bears repeating: Since every plaintext message is equally possible, there is no way for the cryptanalyst to determine which plaintext message is the correct one. A random key sequence added to a nonrandom plaintext message produces a completely random ciphertext message and no amount of computing power can change that. The caveat, and this is a big one, is that the key letters have to be generated randomly. Any attacks against this scheme will be against the method used to generate the key letters. Using a pseudo-random number generator doesn’t count; they always have nonrandom properties. If you use a real random source—this is much harder than it might first appear, see Section 17.14—it’s secure. The other important point is that you can never use the key sequence again, ever. Even if you use a multiple-gigabyte pad, if a cryptanalyst has multiple ciphertexts whose keys overlap, he can reconstruct the plaintext. He slides each pair of ciphertexts against each other and counts the numb...
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