Topic: Why is it more profitable for a node with more than 51% computation power? (Read 171 times)

An attacker with 51% hash power could perform a double-spend (or several). He could send a large payment to someone and then use his hash power to create a longer chain that spends the same coins but sends them to himself instead, making the first transaction invalid.

It would be against his interest because playing by the rules would net him roughly 1 every 2 block rewards (that is roughly $100,000 every 20 minutes) but if he chose to perform this attack the price of bitcoin would plummet making his reward much less.

Let's assume the attacker finds 75 blocks per day. That's currently worth about $6.5 million. If he plays fair, he'll have that income for a very long time, and can use it to earn back the high cost of the hardware he had to buy.

For an attack to be successful, it will need to deliver more than $6.5 million per day on average. I can't really think of a method to quickly cash out or trade thar much money anonymously.

There's this article that says a 51% attack on Ethereum Classic would cost $55 million, and earn you $1 billion profit, but it doesn't say how you can get $1B profit from a coin with just $1.5B market cap. I can't imagine that's possible, as it would mean you have to sell the majority of all coins, and take them back.

You're welcome in advance.

That incentive is the block reward and it's too easy to think of if you're a miner with a city-large mining farm.
Simply Solo mine using that hashrate and you'll definitely get most of the block rewards which is 12.5BTC approximately every 10minutes.

Is there any more profitable choice? 51% attack isn't easy and cheap.

Links to threads about 51% attack that might peek your interest:

• How strong is Bitcoin against 51% attack !
• Can I slowdown someone's transaction?

The whitepaper was written at the time there was only cpu mining. At that time, if you had more than 51% of the network's cpu power, you had more  than 51% of the networks hashrate (depending on your setup and the feature set of the cpu's).
Having more than 51% of the network's hashrate allows you to, statistically, find more blocks by yourself than all other miners pooled together. This would allow you to rewrite the last couple of blocks leaving out transactions that do no benefit you and still end up with the longest chain (the chain that required the most work).

Nowadays, you can no longer mine using your cpu/gpu (well, you *can* technically mine using cpu/gpu, but the odds of solving even a single share are extremely small, so for all practical implications, it's impossible to mine with a cpu/gpu), so the nodes no longer mine.
The theory is still valid tough: if you would own more than 51% of the network's hashrate, you could still do a 51% attack... However, nowadays, you could only gain 51% of thet networks hashrate by investing billions into latest gen ASIC's, this makes a 51% attack extremely costly, and since such an attack would probably decrease bitcoin's price, you'd invest billions and get very little in return (since the "stolen" btc would be worth a lot less). If the community ever discovered somebody was executing a 51% attack, they could *potentially* even decide to fork to a different POW algo, making the attacker's ASIC investment "worthless" (he could still use it to mine sha256d altcoins, but it would be way less profitable than mining BTC).

It is stated in the Bitcoin whitepaper that "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."

I want to know the explanation what can an attacker gain if he has more than 50% computing power and how it is outweighed by what he can gain if he plays by the rules.

Thanks in advance for the clarification.