There are none, even that links that "calculates" the cost is showing only 110petahash available for rent, that's more like  0.1% rather than 51%.

Basically three possibilities to achieve that:
- you buy/build it, and the costs are in billions as per above
- you rent it, but three is nobody renting you that much
- you steal it, as in hijacking a pool or bribing some operator

But all are expensive, they require industry connections, and they all end with the same question, why put all the effort for it? A nation would just take legislative measures, a terrorist group has nothing to earn, other groups are interested in making money not spending money, so just some bored kid who has by chance manage to infect some servers of mining pools but that's quite the stretch.


It's not impossible, yes as I also said the money spending doesn't make sense but it's not impossible. As for the nodes, no, the nodes will not be back to the chain because that would break the rule, they have to follow the chain with the most work, in this care the malicious one.

Exactly what I was thinking when people discuss the cost of a 51% attack on Bitcoin for an hour. However, technically they are somewhat correct, as it seems the available hash rate isn't "freely" available to hire. At least from NiceHash-able (that supplies hashing power) there is not even 1% available. As unsurprisingly there isn't 400 EH/s available to rent, as that is a shitload of hash rate.

As you said you wouldn't be able to bribe mining pools, as they have an interest in the network not getting attacked in such a manner. So trying to convince numerous mining farms to rent you hash power seems like a long-shot to me. At least if a fair rent cost per hour is around $1.3m, then I think it could be up to twice as much in order to convince mining farms to rent you power.

Try to do the 51% attack with money is not the right approach, because if the main mining pools organize to do the attack then the cost would be close to zero. Right now the main pool is Foundry USA with 33.8% of the network, the second one is F2Pool with 17.5% and in 3rd place we have AntPool with 16.9% of the network. So, if Fundry USA buys or collaborates with F2Pool or Antpool, they will have more than 51% of the network and they would be able to do the attack. Simple as that.

But if you think that you can spend billions in miners to get the 51% of the network, that is not possible. There are not enough miners in the market for this.

Unfortunately it can happen. IDK the price of such hustle, but the sum is surely affordable for the worlds main tycoons and banks, especially if their actions will be organised. That's actually the main threat to decentralisation.

I think way back, 51% attack could have happened but the pool operator did not take advantage of the scenario.  Ghash.io briefly exceeded 51% of the hash rate, and that forced the community to discuss a solution to the threat[1]. So, it can be possible in practice if the operator of the huge mining pool agreed to merge their hashes and attack the Bitcoin network.


Or the consensus of different mining operators to join hashes to create a 51% attack on the network.

---

[1] https://en.wikipedia.org/wiki/GHash.io

Well, first my bad at saying 0.1 based on a website that monitors it, if we look at the actual marketplace, yes they do have around 1%, or 3 exas.

Now, there is a trick to this, don't know why I mentioned it first since it was the most important and adding more to Phil's hint at hijacking being the easy way out,  if you rent hashrate from nicehash, it doesn't mean automatically you're free to do anything you want with it, Nicehash for example,has  a policy with limited mining pools to which you can point that, so you will need to either get Nicehash to approve your malicious pool or bribe the pool to which you point this.

So no f#$% way would a big player like mara or core or riot allow you to rent this and point to your pool at Iwannahackbitoin.cheap 
They will rent it to you at a 10% surcharge but only if you mine at either foundry or maybe one or two trustable alternatives.


Yeah, imagine that 10 blocks with like the last block mined that had 7,897.78BTC worth of transactions so around 200 mils, gets invalidated so 2 billion worth of payments deposits, and withdraws suddenly are nulled or are spent differently, multiply this by a bigger timeframe of around 24h and see who will use bitcoin for anything after such an event or how much trust would anyone put in it.

Some are too focused on the aspect of investment where you just buy and hodl and forget that for it to actually have value you need to usage, and this would be sent in a coma if someone would really manage to get such an attack going for a longer period of time, not 1h no 4 h but something in the days value. Highly unlikely but it won't be a walk in the park if it does happen.


No, it's like robbing a bank with a tank and the only guard watching is a guy in the 90s in a wheelchair and armed with a squirt gun.
Since you mentioned this, si a pool does a 51% , you, like the other guys watching, what are you going to do to stop them?
Genuinely curious!

Some other cool, game-theoretic facts about a 51% attack on the Bitcoin network, besides the already mentioned (astronomical) initial costs:
- As soon as the network is attacked, the Bitcoin price would probably drop by a huge factor, so even double spending your own transactions will most likely not make up for your initial costs AT ALL
- You still don't and never will controll any private keys that you didn't already control before the attack
- You can't change previous blocks. If you really control MUCH MORE than 50%, maybe you can afford to even rewrite a few of the latest blocks, but the digital-pyramid that the Bitcoin blockchain is, will have maybe handful of its hundrets-of-thousands of stones changed, the rest will still stand unchanged.