缩写作文一共300词左右,麻烦达人帮我缩写到200词左右,Electronic money (digital cash)

缩写作文
一共300词左右,麻烦达人帮我缩写到200词左右,
Electronic money (digital cash)
Making files unique with digital signatures is the basis of digital cash (electronic money).Alice the banker creates electronic banknotes of various denominations and puts a unique number on every banknote.She signs the result.Bob the client now makes a withdrawal from his account with Alice and receives some of the signed banknotes.The banknotes can be anonymous or include Bob's name.Bob then goes to Charlie's electronic hardware store and purchases a digital camera using these banknotes as payment.Charlie verifies that the banknotes bear Alice's signature and so knows that they are not counterfeit.
Bob could of course make as many copies of the signed banknotes as he wants,since the banknotes are in electronic form.Charlie therefore now has to go to Alice and report to her the unique number on the banknote he received.Alice will then record that number as "spent" and indicate to Charlie that the transaction is okay.If the number was already recorded as "spent",Alice will reject the transaction.If the transaction is okay,the amount indicated on the banknotes is credited to Charlie's account.
This system has many advantages over traditional payment techniques.Alice can create banknotes of any denomination,including for example millicents (0.001 cents).This way for example an electronic archive could charge one millicent for every document Bob requests,and Bob could pay that without having to take a subscription or make a deposit in advance.
One disadvantage of this system is that it requires Charlie to immediately check with Alice whether the banknote he still valid.If Charlie waits even a few minutes,Bob can spend the banknote again at Dave's.Then either Charlie or Dave is not going to get his money.
This principle is currently used for electronic coupons.As a coupon is less valuable than a banknote,the risk of double spending a coupon appears to be acceptable.Furthermore,coupons are usually only valid at one particular store.