51% attack would require a collusion of the large group of the miners, and, if successful, will likely cause a massive price drop. Which is the last thing miners want.
P.S. And btw, it's >50% attack, not 51% attack. You don't need that whole extra percent.
This 51% attack thing is such a canard. If 51% of Bitcoin miners are malicious, the most harm they can do is fail to include your valid transaction in their blocks.
So, your transaction will be confirmed in the next block mined by one of the other 49% of miners. Big whoop.
Yes, that is correct. If it is > 100%, there is enough hashing power available to complete the attack, but you could be outbid during the attack. NiceHash has the concept of fixed contracts that allow you to lock in a price for up to 24 hours and no one can outbid you. Typically ~30% of the hashing power is available this way.
Thanks, I see now, OP's asking about mines, not necessarily miners (though I guess it's a bit hard to distinguish... if I mine, do I have a mine? how many cores should I have to qualify that as a mine?)
Anyway, on the >50% attack, I'm not totally in agreement. The attack can drop transactions, but can't forge any (which, btw, goes towards the "there won't be transactions"). If the network would ever restart, it will be eventually verified.
Whats the point of that ? A miner can confirm non-standard transactions right now when he mines a block but no one would accept it. Point of 51% attack is to surreptitiously double spend and nothing else.
But. A 50% attack has the problem that an attacker with enough hash power to do it has enough hash power to simply mine on chain legitimately and create blocks and get 50% of the block reward and make tons of money that way, while actually doing the attack decreases confidence in the chain and might end up with massive losses in price.
Yes it is true. If a miner has 51% hash rate, they can in theory steal your coins by rewriting history to replace the transaction that granted you the coins with a different transaction that sends the coins somewhere else. Or, if your coins are too old for that to work, they can DoS you by refusing to mine on any block that includes your transaction, essentially rendering your bitcoins worthless.
In practice these attacks probably need significantly more than 51% hash rate, but there is some % hash rate where they would work. Of course, evidence that such an attack was being carried out would immediately crash the Bitcoin market, which would destroy the value of the miner's investment in mining hardware. (A crude form of proof of stake?) So it's hard to imagine that such an attack would actually happen in reality, unless it was carried out by someone with the intent to destroy Bitcoin and a few billion dollars to burn.
Miners only control transaction inclusion/ordering. That's the only thing that a 51% attack will grant you. You still can't forge coins or make unauthorized transfers. You can do double spend attacks and tank the trust of the network, but I'm skeptical whether that's actually worth it. It's probably cheaper to send death squads to users/merchants/exchanges.
There is even some evidence that double-spending attacks have already happened. You don't strictly speaking need 51%, that majority just ensures that your chain will win out over time. The original bitcoin paper[1] details the probabilities involved for nodes controlling < 51%, if anyone is interested it is a short and good read.
It could be even worse than that however. Another post on BCT[2] talks about a "time warp attack", where at 51% node could effectively mine all remaining bitcoins in one fell swoop.
No they won't. As the cost of 51% attack is actually the cost of fee of cancelled transaction(and subsequent ones) plus created coins. The attacker can collect all the fees also from those blocks they mined for attack...
Ofc, if the mining is sufficiently large that it is impossible for anyone to collect enough power, then sure. But that is quite expensive proposition in general...
Technically, a 51% attack would allow one to confirm non-standard transactions and pretty much do anything. Granted, this would immediately be apparent to any of the other 49% of nodes and cause a fork, but still...
Thanks for the additional technical details. But I don't think it changes anything.
> Bitcoin does not allow these kind of transactions to be used as input for another transaction unless a certain number of confirmations has passed.
This means that in the hypothetical attack case, where 1008 blocks are replaced by 1009 blocks by the 51% attacker... then 909 x 12.5 BTC are immediately usable. While the last 100 x 12.5 BTC will be usable in 1000 minutes later.
In either case, the 51% attacker mines blocks faster than literally everyone else combined. So there's no period where of waiting that is safe against a 51% attacker.
The 51% attack is the end-all-be-all of a coin. If it happens, the coin is hopelessly lost. The entire cryptographical nature of the coin depends on the 51% attack remaining infeasible.
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In any case, the 51% attacker's 1009 blocks is longer than the 1008 "honest blockchain". So the honest miners (by default) will mine the 1009-long blockchain after the 51% attack.
So the 51% attacker's coins are safe and will be spendable in short order. Perhaps the 49% honest miners can "blacklist" his coins, but with 51% of the hashrate, the 51% attacker can always legitimize his own coins by using his own 51% hardware (since he can build blocks faster than everyone else).
This is true. A 51% attack in which the attacker prevents all transactions from going through is probably bitcoin's largest vulnerability.
However, even at today's difficulty levels, this attack would be very expensive to maintain. With the introduction of dedicated mining ASIC's coming soon, the difficulty will rise substantially, which will offer even more security against this type of attack.
You'll need 100%, because if a single miner decided to include the transaction, then it is in!
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