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Unformatted text preview: to sign the last check (if Alice is caught cheating) without penalizing Alice, she will continue to try until she gets lucky. Prison terms are a better deterrent. Protocol #2
The previous protocol prevents Alice from writing a money order for more than she claims to, but it doesn’t prevent Alice from photocopying the money order and spending it twice. This is called the double spending problem; to solve it, we need a complication: (1) Alice prepares 100 anonymous money orders for $1000 each. On each money order she includes a different random uniqueness string, one long enough to make the chance of another person also using it negligible. (2) Alice puts one each, and a piece of carbon paper, into 100 different envelopes. She gives them all to the bank. (3) The bank opens 99 envelopes and confirms that each is a money order for $1000. (4) The bank signs the one remaining unopened envelope. The signature goes through the carbon paper to the money order. The bank hands the unopened envelope back to Alice and deducts $1000 from her account. (5) Alice opens the envelope and spends the money order with a merchant. (6) The merchant checks for the bank’s signature to make sure the money order is legitimate. (7) The merchant takes the money order to the bank. (8) The bank verifies its signature and checks its database to make sure a money order with the same uniqueness string has not been previously deposited. If it hasn’t, the bank credits $1000 to the merchant’s account. The bank records the uniqueness string in a database. (9) If it has been previously deposited, the bank doesn’t accept the money order. Now, if Alice tries to spend a photocopy of the money order, or if the merchant tries to deposit a photocopy of the money order, the bank will know about it. Protocol #3
The previous protocol protects the bank from cheaters, but it doesn’t identify them. The bank doesn’t know if the person who bought the money order (the bank has no idea it’s Alice) tried to chea...
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- Fall '10