According to the paper, each user of the system could have one or more public Bitcoin addresses—sort of like bank account numbers—and a private key for each address. The coins attached to a given address could be spent only by a person with the private key corresponding to the address. The private key was slightly different from a traditional password, which has to be kept by some central authority to check that the user is entering the correct password. In Bitcoin, Satoshi harnessed the wonders of public-key cryptography to make it possible for a user—let’s call her Alice again—to sign off on
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