51% attack: 51% of what?

[jfhoff]

We hear a lot about the possibility of a 51% (more accurately, 50%+) attack on BTC, but I don't understand what the attack requires 51% of. Is it the miners, and if so, is it 51% of the total hash rate? How is that determined and propagated? Do I then, as a BTC holder but not miner, have no influence on this? Or is it somehow tied to the total number of BTC in circulation?

[RodeoX]

Basically, if someone created a fake block and convinced more than 50% of miners that it was real, then it would be included in the block chain and create a fork in the chain. It would take HUGE computational power to "convince" the network that it was a real block.

So, 51% of the network hashing power.

[jfhoff]

So let's say a group of nefarious miners consorted to place a fake block on the chain. Then there would be two blocks to follow... how do miners proceeding to the next block choose which fork to follow? When there are two possibilities, it would seem that it would be trivial for every miner to notice that there are two options and recalculate each one to select the correct one... oh yeah, half the minors (the nefarious ones) would select the fake block, and so on, eventually overpowering the true block.

Right?

[Gabi]

It is easy, if you have the majority of the hashing power, you can do what you want with the blockchain, because your will be longer. You can just make new blocks and refuse every other, maybe including only the transactions you want. You can even rebuild the blockchain, for example you can start from 6 months ago, rebuild it as you wish to today. Your chain will always be the "longer" (note that with "longer" the difficulty is also included, 1 block at difficulty 1000 is "longer" than 10 at difficulty 10) because you hold more than half of the total hashpower.

[RodeoX]

If I understand you correctly, then yes. All it takes is a majority of miners to forge a block and willfully put it in the blockchain. But why would they do that? It would likely severely hurt the price of bitcoin and it would cost vast amounts of money for the needed electricity.  It would be even harder for an outside attacker, as they would need to beat the vast computational power of the network.

[Gabi]

But why would they do that? It would likely severely hurt the price of bitcoin and it would cost vast amounts of money for the needed electricity.

A government/bank/entity attacking bitcoin to protect their ability to print trillions of dollars obviously doesn't care about spending some millions for this attack

It would be even harder for an outside attacker, as they would need to beat the vast computational power of the network.

Nah, some millions of dollars would be enough. Maybe some dozens. Basically what the FED print in a day or so.

[jfhoff]

This kind of attack, by a central bank or a large government, is the one that worries me. Even though such an attack requires a "HUGE" amount of computing power, it would be trivial for a large, threatened institution that is concerned, not in turning a buck, but in destroying BTC.

[RodeoX]

I am not worried about an attack from the federal reserve. That is not what they would do. Despite the many conspiracy theorists here, they are accountable and do not engage in attacking anyone. Theoreticly a board of directors of a bank could decide to attack bitcoin. They would have to spend millions of dollars and somehow elicit the power of many/most of the worlds supercomputers. Of course anyone could jsut create bitcoin 2.0 and the bank would have accomplished nothing. It is not financially viable to attack bitcoin, and banks ONLY care about financial viability. They are not stupid. It is why they are holding more of your money than you are.

[DeathAndTaxes]

This kind of attack, by a central bank or a large government, is the one that worries me. Even though such an attack requires a "HUGE" amount of computing power, it would be trivial for a large, threatened institution that is concerned, not in turning a buck, but in destroying BTC.

I would think most governments realize the futility of this.  I mean the destruction of napster killed file sharing right?  Also unlike napster no government has made virtual currencies illegal.  Lots of US citizens for example hold legal Bitcoins, the US government willfilly destroying legal property of its own citizens would be ground for a lawsuit against the government.  Also no government is an island unto itself.  There would be international legal and foreign policy fallout.  If citizens in other countries lost funds there could be trade retaliation, demands for compensation, etc.

Now someone might naively think the US government could keep this a secret but really look at recent US screw ups ("Benghazi" anyone) and all the embaressing details which leaked out.

Still in theory it could happen however it would simply push crypto-currency into other methods of securing the blockchain like POW & POS hybrids or even more advanced concepts like proof of history (strongly signed miners), etc.  One could even take the existing blockchain prior to the attack and use it as the genesis block to bootstrap a new crypto-currency with hundreds of thousands of users on day 0.

It could happen but I don't lose much sleep over it.

[jfhoff]

RodeoX, I think you are overestimating the computing power needed to take over the block chain, and underestimating the threat that BTC poses to the established financial community, both fiat currency providers and credit card/POS issuers.

The current hash rate is 63.686 THh/s, and a production mining rig does 50 GH/s. So with a mere 1200 commercial rigs, the entire current hash rate can be duplicated.

Do you think that these established players will let themselves be brought down when the cost is so small to bring BTC down?

BTW, I am an ASIC designer by trade, and have worked on the most powerful computers, used by Livermore Labs. I know what I'm talking about technically. It would be trivial.

[jfhoff]

DeathAndTaxes, if methods exist to secure the blockchain from attack, they should be implemented now, not after the established financial community realizes the threat and reacts.

[RodeoX]

RodeoX, I think you are overestimating the computing power needed to take over the block chain, and underestimating the threat that BTC poses to the established financial community, both fiat currency providers and credit card/POS issuers.

The current hash rate is 63.686 THh/s, and a production mining rig does 50 GH/s. So with a mere 1200 commercial rigs, the entire current hash rate can be duplicated.

Do you think that these established players will let themselves be brought down when the cost is so small to bring BTC down?

BTW, I am an ASIC designer by trade, and have worked on the most powerful computers, used by Livermore Labs. I know what I'm talking about technically. It would be trivial.

I see what your saying, but what would be accomplished by buying 1200 mining rigs, then set them to work destroying mining? If they did such a crazy thing and committed to forking the chain and destroying bitcoin at great expense, I could create bitcoin 2. How many times are they going to waste their money?

[deadweasel]

RodeoX, I think you are overestimating the computing power needed to take over the block chain, and underestimating the threat that BTC poses to the established financial community, both fiat currency providers and credit card/POS issuers.

The current hash rate is 63.686 THh/s, and a production mining rig does 50 GH/s. So with a mere 1200 commercial rigs, the entire current hash rate can be duplicated.

Do you think that these established players will let themselves be brought down when the cost is so small to bring BTC down?

BTW, I am an ASIC designer by trade, and have worked on the most powerful computers, used by Livermore Labs. I know what I'm talking about technically. It would be trivial.

It would not be trivially expensive.  As in, the cost to do such an attack is not worth the the exceedingly small dent that BTC has made in the established financial community.  

Also, if they are so very established, they may believe BTC doesn't stand a chance.  It's just 1's and 0's, they may not get it.

If they are established and technically savvy, they would more likely find a way to PROFIT off of it rather than take on a HUGE expense (1200 50GH miners) just for the possibility to double-spend some bitcoins....

If they're in it for the money, they will not take a loss.  But they might take as much profit as they can while making BTC unstable and ugly to investors.

[jfhoff]

I see what your saying, but what would be accomplished by buying 1200 mining rigs, then set them to work destroying mining? If they did such a crazy thing and committed to forking the chain and destroying bitcoin at great expense, I could create bitcoin 2. How many times are they going to waste their money?

You're right, the concept of stateless crypto-currency is out of the bag, and there is likely no way to put it back in. But I'd hate to see all the people that have invested time, sweat and blood into BTC v1 lose all, and a new crop of investors (the status quo financial crowd?) start up BTC v2, with them reaping all the rewards.

[DeathAndTaxes]

DeathAndTaxes, if methods exist to secure the blockchain from attack, they should be implemented now, not after the established financial community realizes the threat and reacts.

Bitcoin works on consensus, there will never be any changes to the core functionality of Bitcoin.  If you are hyper concerned then research alt-coins, possibly even fund the development of more secure alternatives.

[DeathAndTaxes]

The current hash rate is 63.686 THh/s, and a production mining rig does 50 GH/s. So with a mere 1200 commercial rigs, the entire current hash rate can be duplicated.

That is why ASICs are being developed.  You are comparing the network mostly of GPU/CPU to the potential hashing power of a next generation device.   Once ASICs are being delivered in large quantities the network hashpower is going to grow significantly.  10x to 20x in the near future isn't that unlikely.  Hashrate will probably rise by a factor of 50x and that is without further rise in exchange rate.  As exchange rate rises hashpower will rise in line with it.  So in 2-3 years you could be looking at more like 6,000 to 10,000 TH/s.

[jfhoff]

That increase may be enough to save BTC. Let's hope. But it must occur before the financial community switches from viewing BTC as a "bubble" and sees their power being ripped out from under them.