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In the bitcoin wiki article on Majority attack (https://en.bitcoin.it/wiki/Majority_attack) it says: "No amount of confirmations can prevent this attack; however, waiting for confirmations does increase the aggregate resource cost of performing the attack, which could make it unprofitable [...]"

Taking into account that the attacker miner will impose his block-chain branch (he has over 50% of hashrate) eventually, won't the bitcoin reward for mining these blocks make up for this aggregate resource cost, thus making the wiki statement false? Am I missing anything (I probably do)?

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One thing to consider is that if tomorrow some attacker dumps his year long alternative chain on us, basically reversing a years worth of confirmations: that will likely cause panic and would cause a big drop in price of Bitcoin. So even though he got a lot of rewards in his blocks, if the price halves, the rewards will be worth less than if he had just used his mining power honestly to begin with.

Another thing to consider is that it would be fairly easy for the community to decide to completely invalidate the attacker's chain. For example by blacklisting the hash of his first block in a hard coded manner. Yes that is a centralized decision and yes that will still be a very messy and damaging situation where some wallets get confused and pick the wrong chain. But it is possible to make the attacker's chain completely worthless and should be therefore be a deterrent against such attacks (with long chains at least).

  • Yes, but in a case of double-spending the attacker should only dump a chain as long as the number of confirmations the merchant/provider waits. And that wouldn't be a year long. – Marc Cayuela Rafols Feb 26 '16 at 8:21

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