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

Applied cryptography protocols algorithms and source code in c

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Unformatted text preview: cannot make the connection, even if it keeps complete records of every transaction. Even more strongly, there is no way for the bank and the merchant to get together to figure out who Alice is. Alice can walk in the store and, completely anonymously, make her purchase. Eve can cheat. If she can eavesdrop on the communication between Alice and the merchant, and if she can get to the bank before the merchant does, she can deposit the digital cash first. The bank will accept it and, even worse, when the merchant tries to deposit the cash he will be identified as a cheater. If Eve steals and spends Alice’s cash before Alice can, then Alice will be identified as a cheater. There’s no way to prevent this; it is a direct result of the anonynimity of the cash. Both Alice and the merchant have to protect their bits as they would paper money. This protocol lies somewhere between an arbitrated protocol and a self-enforcing protocol. Both Alice and the merchant trust the bank to make good on the money orders, but Alice does not have to trust the bank with knowledge of her purchases. Digital Cash and the Perfect Crime Digital cash has its dark side, too. Sometimes people don’t want so much privacy. Watch Alice commit the perfect crime [1575]: (1) Alice kidnaps a baby. (2) Alice prepares 10, 000 anonymous money orders for $1000 (or as many as she wants for whatever denomination she wants). (3) Alice blinds all 10, 000 money orders, using a blind signature protocol. She sends them to the authorities with the threat to kill the baby unless the following instructions are met: (a) Have a bank sign all 10, 000 money orders. (b) Publish the results in a newspaper. (4) The authorities comply. (5) Alice buys a newspaper, unblinds the money orders, and starts spending them. There is no way for the authorities to trace the money orders to her. (6) Alice frees the baby. Note that this situation is much worse than any involving physical tokens—cash, for example. Without physical contact, the...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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