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

Each customer has an account at two different banks

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Unformatted text preview: police have less opportunity to apprehend the kidnapper. In general, though, digital cash isn’t a good deal for criminals. The problem is that the anonymity only works one way: The spender is anonymous, but the merchant is not. Moreover, the merchant cannot hide the fact that he received money. Digital cash will make it easy for the government to determine how much money you made, but impossible to determine what you spent it on. Practical Digital Cash A Dutch company, DigiCash, owns most of the digital cash patents and has implemented digital cash protocols in working products. Anyone interested should contact DigiCash BV, Kruislaan 419, 1098 VA Amsterdam, Netherlands. Other Digital Cash Protocols There are other digital cash protocols; see [707, 1554, 734, 1633, 973]. Some of them involve some pretty complicated mathematics. Generally, the various digital cash protocols can be divided into various categories. On-line systems require the merchant to communicate with the bank at every sale, much like today’s credit-card protocols. If there is a problem, the bank doesn’t accept the cash and Alice cannot cheat. Off-line systems, like Protocol #4, require no communication between the merchant and the bank until after the transaction between the merchant and the customer. These systems do not prevent Alice from cheating, but instead detect her cheating. Protocol #4 detected her cheating by making Alice’s identity known if she tried to cheat. Alice knows that this will happen, so she doesn’t cheat. Another way is to create a special smart card (see Section 24.13) containing a tamperproof chip called an observer [332, 341, 387]. The observer chip keeps a mini database of all the pieces of digital cash spent by that smart card. If Alice attempts to copy some digital cash and spend it twice, the imbedded observer chip would detect the attempt and would not allow the transaction. Since the observer chip is tamperproof, Alice cannot erase the mini-database without permanently damaging the smart card. The cas...
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