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White paper:

"The incentive may help encourage nodes to stay honest. If a greedy attacker is able to assemble more CPU power than all the honest nodes, he would have to choose between using it to defraud people by stealing back his payments, or using it to generate new coins. He ought to find it more profitable to play by the rules, such rules that favour him with more new coins than everyone else combined, than to undermine the system and the validity of his own wealth."

Still, I've read bitcoin community is concerned with 51% attack:

http://blogs.wsj.com/moneybeat/2014/06/16/bitbeat-a-51-attack-what-is-it-and-could-it-happen/

Actual bitcoin implementation does or does not have incentive system as per whitepaper to discourage 51% attack?

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Bitcoin has a huge incentive system to discourage a 51% attack. Bitcoin mining with CPUs is impractical. You have to use dedicated hardware that can only be used to mine Bitcoin and other coins that use the exact same algorithm.

So to acquire enough hashing power to launch a 51% attack on Bitcoin, you can't just rent computing power or buy CPUs. You have to invest in Bitcoin mining equipment. If you manage to harm Bitcoin significantly, you're just turning your expensive mining hardware into so many doorstops.

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  • I understand everything else except the last "doorstops" metaphor. What does it mean here? It looks to me like if an attacker manages to harm Bitcoin, people would stop using and trusting it, so the price would drop, and the attacker may hardly earn enough to pay back his investment into the hardware. Am I understanding right? Or did you mean something else? (BTW I'm questioning myself when I said "the price would drop" because recently the bitcoin to US dollar price had already dropped so much.)
    – yaobin
    Commented Oct 17, 2018 at 23:06
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    @yaobin That's correct. Because bitcoin's mining algorithm runs best on ASICs that can't do anything else, to be in a position to attack the bitcoin network, you first have to invest in it. One wrinkle is that there is now more than one blockchain (bitcoin and bitcoin cash) that can use the same ASICs.
    – David Schwartz
    Commented Oct 17, 2018 at 23:14
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This incentive is not of a technical nature, so it can not be "built in". It depends on the assumption that potential attackers are rational and that their goal is to maximize their monetary gains. The bitcoin community still fears a 51℅ attack because it is conceivable that an attacker is driven by other motives (i.e. wants to destroy Bitcoin) or is behaving irrationally.

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I think the incentive is mining and transaction fees. The system is designed so it's way more profitable to mine and regularly get transaction fees than try a massive attack against 51% of the entire bitcoin network in order to rewrite portions of the blockchain

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I think that even after a succesful 51% atack the network could reach a consensus to ignore it and start again from the last good chain, maybe with a new hash algorithm to make the atacker hardware useless.

If anyone can verify the blockchain there is no reason to throw away years of transantions just because a few blocks are crap.

So I think that is the biggest unincentive, If you break the rules with a 51% atack why would people follow the rule of accepting the longest chain?

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