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See also What can an attacker with 51% of hash power do?

This may have been discussed elsewhere, but I'm not sure what the answer is - if someone owns 51% of the hash power, can't he just ignore the rest of the network, and only focus on his chain, thus earning more than 51% of the rewards (100% in fact)? Will he in fact earn 100% of the reward?

Also - is it possible that several attackers working independently could achieve this scenario? If two people have 34% each, won't one of them always out-race the legitimate chain? Of course, they can't both win, but for some period of time, won't the stronger one among them gain more than his fair share of block rewards?

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  • Interesting point. I never realized you could go from 50% of the block rewards to suddenly 100% simply by orphaning all blocks from competing miners.
    – user220
    Commented Oct 11, 2011 at 9:24
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    @WimCoenen - it's not a sudden increase, see theymos' answer and my comments.
    – ripper234
    Commented Oct 11, 2011 at 9:45

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If you have 51% of the network and you see that someone else has solved a block, then you can either accept their block or compete with it. In either case, if you get the next block you'll get 50 BTC. Usually neither option is more desirable than the other unless you're doing other attacks. It might be a bit better to take 100% if you're worried about difficulty or subsidy adjustments.

So an attacker could get 100% of block rewards, but this alone won't help him much.

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  • I guess that depends how long your attack runs. If your attacks runs longer than the time needed for difficulty adjustments, then the next adjustment will only see your (shorter) chain, and the block reward you receive should increase if you ignore other blocks. Am I right?
    – ripper234
    Commented Oct 11, 2011 at 9:41
  • Actually, isn't is always profitable to do this, if you're fairly certain that you do have 51%? (If you're not certain of it, the you're at risk of losing all your rewards if the other chain gets longer).
    – ripper234
    Commented Oct 11, 2011 at 9:45
  • @ripper234 You can decrease difficulty to some degree. Maybe this would be profitable.
    – theymos
    Commented Oct 12, 2011 at 2:11
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    If you mine legitimately, you get 51% of the block rewards, or about 50 Bitcoins per 20 minutes. If you ignore all other blocks, you get 100% of the block rewards, or about 50 Bitcoins per 10 minutes. It's a no brainer.
    – David Schwartz
    Commented Oct 12, 2011 at 12:51
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Yes, if you have 51% of the network you can reject all other blocks, thus making sure you receive all rewards. The difficulty will adjust to match just your own hashrate, making sure you mine the expected 144 blocks per day.

In fact, with a clever strategy you can get more than your fair share of block rewards even if you're below 50%. This was discussed here - for example, with 46% of the hashrate you can get 51.29% of the rewards, assuming the rest of the network is honest.

In the multi-attacker case, an attacker with more hashrate than the honest network and any other attacker (where every attacker rejects all blocks but his own), will get 100% of the blocks. If there are several attackers at the top, you'll have occasional massive reorgs where one moment one attacker owns all the bitcoins generated since the attack started (if he hasn't spent them), and the next moment another attacker one-ups him and replaces the whole branch with one where he has generated all coins.

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He could do whatever he wants to, the problem is legitimate clients would ignore the blocks if they are out-of-spec!

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    Why do you think the blocks he generated would be "out-of-spec"?
    – ripper234
    Commented Oct 11, 2011 at 8:47
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    The client will follow the rules and use the longest chain. So if attacker has 51% of hashing power and doesn't give up (i.e. abandon current block when a legit one is found) eventually the odds are in favor of the attacker that he will have a longer chain. With 51% vs 49% the cheating attacker will have the longest chain 99% of time after 10 blocks. At which point every client on the network will see the attacker's chain as the longer and thus valid one.
    – DeathAndTaxes
    Commented Oct 20, 2011 at 20:44

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