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could spend or take the money associated with a particular Bitcoin address. What’s more, each user of the system could be confident that, at every moment in time, there would be only one public, unalterable record of what everyone in the system owned. To believe in this, the users didn’t have to trust Satoshi, as the users of DigiCash had to trust David Chaum, or users of the dollar had to trust the Federal Reserve. They just had to trust their own computers running the Bitcoin software, and the code Satoshi wrote, which was open source, and therefore available for everyone to review. If the users didn’t like something about the rules set downby Satoshi’s software, they could change the rules. People who joined the Bitcoin network were, quite literally, both customers and owners of both the bank and the mint.
    But so far, at least, all Satoshi had done was describe this grand scheme.
    D ESPITE ALL THE advances described in the Bitcoin paper, a week after it was posted, when Hal Finney chimed in for the first time, there were only two responses on the cryptography mailing list. Both were decidedly negative. One noted computer security expert, John Levine, said that the system would be easily overwhelmed by malicious hackers who could spread a version of the blockchain that was different from the one being used by everyone else.
    â€œThe good guys have vastly less computational firepower than the bad guys,” Levine wrote on November 2. “I also have my doubts about other issues, but this one is the killer.”
    Levine’s concern was a valid one. The Bitcoin system Satoshi described relied on computers reaching decisions by majority rule. Early on, when there were fewer computers on the network, it would be easier to become the majority and take over. But Satoshi’s hope was that there wouldn’t be much of an incentive to take over the system early on, when the network was small. Later on, if there was an incentive to attack the network, that would hopefully be because the network had attracted enough members to make it hard to overwhelm.
    Another longtime veteran of the Cypherpunk debates, James Donald, said that “we very, very much need a system,” but the way he read the paper, the database of transactions, the blockchain, would quickly become too big for users to download.
    In the weeks that followed, Hal was essentially Satoshi’s only defender. On the cryptography list, Hal wrote that he wasn’tterribly worried about the attackers that Levine talked about. But Hal admitted that he wasn’t sure how the whole thing would work in practice, and expressed a desire to see actual computer code, rather than just a conceptual description.
    â€œThis does seem to be a very promising and original idea, and I am looking forward to seeing how the concept is further developed,” Hal wrote to the group.
    Hal’s defense of the program led Satoshi to send him an early, beta version for testing. In test runs in November and December they worked out some of the early kinks. Not long after that, in January 2009, Satoshi sent the complete code to the list. The final software made some interesting tweaks to the system described in the original paper. It determined that new coins would be assigned approximately every ten minutes, with the hash function lottery getting harder if computers were generating coins more frequently than that.
    The software also mandated that the winner of each block would get fifty coins for the first four years, twenty-five coins for the next four years, and half as much again every four years until 21 million coins were released into the world, at which point new coin generation would stop.
    On the first day, when Hal downloaded the software, the network was already up and running. For the next few days, not much activity was being added to the blockchain other than a computer on the network (usually belonging to Satoshi) winning fifty coins