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applied cryptography - protocols, algorithms, and source code in c

# The bank xors the two halves together to reveal

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Unformatted text preview: : Protocols, Algorthms, and Source Code in C (cloth) Go! Keyword 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: Go! Previous Table of Contents Next ----------- This protocol assumes that the merchant cannot change the identity string once Alice writes it on the money order. The money order might have a series of little squares, which the merchant would require Alice to fill in with either Xs or Os. The money order might be made out of paper that tears if erased. Since the interaction between the merchant and the bank takes place after Alice spends the money, the merchant could be stuck with a bad money order. Practical implementations of this protocol might require Alice to wait near the cash register during the merchant-bank interaction, much the same way as credit-card purchases are handled today. Alice could also frame the merchant. She could spend a copy of the money order a second time, giving the same identity string in step (7). Unless the merchant keeps a database of money orders it already received, he would be fooled. The next protocol eliminates that problem. Protocol #4 If it turns out that the person who bought the money order tried to cheat the merchant, the bank would want to know who that person was. To do that requires moving away from a physical analogy and into the world of cryptography. The technique of secret splitting can be used to hide Alice’s name in the digital money order. (1) Alice prepares n anonymous money orders for a given amount. Each of the money orders contains a different random uniqueness string, X, one long enough to make the chance of two being identical negligible. On each money order, there are also n pairs of identity bit strings, I1, I2,..., In. (Yes, that’s n different pairs on each check.) Each of these pairs is generated as follows: Alice creates a string that gives her name, address, and any other piece of identifying information that the...
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