51% attack

[jonald_fyookball]

The real danger is not in someone taking over 50% of the existing network, but rather someone getting a hold of enough hashing power to double the existing network to create the 51% attack.  That is a dire threat , but I'm not sure what is being done.

[birkomester]

This will not happen.
Why would anyone do this? They would only lose money.

[jonald_fyookball]

To destroy bitcoin.

[Lauda]

The real danger is not in someone taking over 50% of the existing network, but rather someone getting a hold of enough hashing power to double the existing network to create the 51% attack.  That is a dire threat , but I'm not sure what is being done.

This isn't going to happen if the few right people care enough. Miners should spread out to more pools because of this. It is solely in their hands.

[jonald_fyookball]

its irrelevant what pools people join if a big bank or government decides to invest a few hundred million (or less)
to develop their own 60 Ph/s of mining power.

[Lauda]

its irrelevant what pools people join if a big bank or government decides to invest a few hundred million (or less)
to develop their own 60 Ph/s of mining power.

This was discussed before. Why would anyone with any amount of brain waste so much power just to 51% attack it, when the coin can be revived afterwards? The more miners we have, the less chance the government has of overtaking us with the mining power. The improvement in ASICs will stagnate one day.

[jonald_fyookball]

how would it be revived afterward?

[Lauda]

how would it be revived afterward?

Many alt coins suffered from 51% attacks and survived. Go do your research.

[jonald_fyookball]

As long as they have 51 of the hashing power , they can control which transactions to accept.  Even blocking a small amount of transactions will destroy credibility of bitcoin.

I've seen a lot of "problems" with bitcoin brought up by people in the forum that are non issues.  This does appear to be something of concern.

[Brangdon]

This was discussed before. Why would anyone with any amount of brain waste so much power just to 51% attack it, when the coin can be revived afterwards?

The goal would be to destroy Bitcoin. I don't think they'd bother now, but as it becomes more successful it becomes more of a target, until (in my view) a large attack is inevitable.

Why do you think Bitcoin could be revived afterwards? Why would there even be an "afterwards"? Once they've bought the hardware, they can continue the attack indefinitely, for only the cost of electricity. They don't even need to run it continuously. They could, at random times, release a few hours of privately mined forked chain. That would be enough to make the system almost unusable.

The more miners we have, the less chance the government has of overtaking us with the mining power. The improvement in ASICs will stagnate one day.

By "the government", are you thinking America is the only threat? What about terrorists? As an attack on financial infrastructure, it would have similar symbolism to the 9/11 attack on the World Trade Centre. (How can you get more "world trade" than Bitcoin?) It would avoid the negative publicity of civilian deaths, and avoid the need for them to risk their own deaths. Alternatively, Russia might do it in retaliation for economic sanctions imposed on them, eg over Ukraine. Or China, or any country that has currency controls. Or a private bank. HSBC recently paid over $4b in fines; $100m would be easily affordable to them.

ASICs will plateau, but the block-chain reward will also decrease, and I don't think transaction fees will rise quickly enough to compensate, and nor will the volume of transactions. So we may find that existing miners keep going, because their ASICs are good for nothing else, but few new miners will want to get involved because they'll never cover their capital costs. And while the attacker is attacking, the miners could be mining zero blocks.

[Stuartuk]

Interesting topic. I'd just throw into the mix the possibility or targeted attacks on the infrastructure of the largest farms. If the power supplies of the top five farms suffered a coordinated global attack what percentage of the network could be taken down? If it were a significant amount then it would be worth doing just before a 51% attack as the hardware then needed to get to 51% of the reduced network would be significantly smaller.

Would such a 51% attack fail once the farms got their power back up and that 51% became 40%? If so a more permanent shut-down could attempted eg fire/explosive attack on the largest farms.

[Peter R]

its irrelevant what pools people join if a big bank or government decides to invest a few hundred million (or less)
to develop their own 60 Ph/s of mining power.

How would a government do this, Jonald?  Would Congress pass the "51% Attack Bitcoin Bill" authorizing spending of $0.5 billion?  Do you think such a bill would pass, let alone receive popular support?  

If your answer is that a small-subset of government would do it secretly and in private, then why would they bother 51% attacking bitcoin?  Why not just pocket all the mined bitcoins if they knew they wouldn't get caught.  "Oops, I guess it didn't work.  Good thing the tax payers paid for these bitcoin miners.  Hey, what happened to those mined coins?"

And it's not like the 51% attack would kill bitcoin anyways.  It would just be annoying for a while (and a bunch of people would probably panic sell).  

As for banks, if they thought bitcoin was such a threat that it was worth spending half a billion just to annoy the network for a while, then why not invest half a billion dollars into purchasing bitcoin for cheap, perhaps from a bankrupt exchange, co-opt the system, and pump the price to the moon?  The most talented people at the big banks are well situated to profit from bitcoin.

That all being said, I do support efforts to decentralize mining!

[jonald_fyookball]

its irrelevant what pools people join if a big bank or government decides to invest a few hundred million (or less)
to develop their own 60 Ph/s of mining power.

How would a government do this, Jonald?  Would Congress pass the "51% Attack Bitcoin Bill" authorizing spending of $0.5 billion?  Do you think such a bill would pass, let alone receive popular support?  

If your answer is that a small-subset of government would do it secretly and in private, then why would they bother 51% attacking bitcoin?  Why not just pocket all the mined bitcoins if they knew they wouldn't get caught.  "Oops, I guess it didn't work.  Good thing the tax payers paid for these bitcoin miners.  Hey, what happened to those mined coins?"

And it's not like the 51% attack would kill bitcoin anyways.  It would just be annoying for a while (and a bunch of people would probably panic sell).  

As for banks, if they thought bitcoin was such a threat that it was worth spending half a billion just to annoy the network for a while, then why not invest half a billion dollars into purchasing bitcoin for cheap, perhaps from a bankrupt exchange, co-opt the system, and pump the price to the moon?  The most talented people at the big banks are well situated to profit from bitcoin.

That all being said, I do support efforts to decentralize mining!

It could be part of the defense budget, who knows...maybe JP Morgan just decides to do it...or HSBC...or both... I have no idea.  I trust government and banks about as far as I can throw them.  My point is, lets make as technically infeasible as possible.  And right now its technically feasible. 

And I would argue that a sustained 51% attack (mining monopoly) could utterly destroy confidence in Bitcoin if the monopoly was malicious.

[Peter R]

It could be part of the defense budget, who knows...maybe JP Morgan just decides to do it...or HSBC...or both... I have no idea.  I trust government and banks about as far as I can throw them.  My point is, lets make as technically infeasible as possible.  And right now its technically feasible.  

And I would argue that a sustained 51% attack (mining monopoly) could utterly destroy confidence in Bitcoin if the monopoly was malicious.

We are talking in 2 threads simulateously, lol.  I explained how even the extremely unlikely event of a sustained 51% attack is not that great a threat here: https://bitcointalk.org/index.php?topic=581635.msg6364725#msg6364725 .  

The value of bitcoin comes from our community's shared agreement that the blockchain ledger is legitimate.  As long as this is the case, bitcoin will live on.  

[tzortz]

Interesting topic. I'd just throw into the mix the possibility or targeted attacks on the infrastructure of the largest farms. If the power supplies of the top five farms suffered a coordinated global attack what percentage of the network could be taken down? If it were a significant amount then it would be worth doing just before a 51% attack as the hardware then needed to get to 51% of the reduced network would be significantly smaller.

Would such a 51% attack fail once the farms got their power back up and that 51% became 40%? If so a more permanent shut-down could attempted eg fire/explosive attack on the largest farms.

An explosion would be great at Knc's farm.

[porcupine87]

Here, what a user with 51% can do and not do:
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power

https://www.youtube.com/watch?v=bi2thGzzNSs

seems like it's alot easier then some people claimed...

in theory.. you dont need to buy thousands of ASICS.

you just need to bribe the top 3 mining pool owners to collectively use a different mining script that you have coded.

bribing 3 mining pool owners is much cheaper then buying the equipment

Oh, I never thought of that. So the 51% attack is an issue. We need to trust the pool operators. I hope P2P pools gain in hashing power. Does anyone know how much they do nowerdays?

IF a 51% attack truly did happen then Litecoin could just step up to the #1 slot. Then if a 51% attack happened to Litecoin (Completely different hardware required) then a CPU based coin like Primecoin would step up.
etc..
Not gonna happen.

No altcoin with POW is immune against that 51%. Not matter whether GPU or CPU mining. So POS coins could be an alternative.

To prevent 51% attack, someone has to mass produce low power asic at low cost.
So when every bitcoin user are solomining, we can easily achieve a crazy hashrate deterring 51% attacks.

You think, someone has that capital so buy such mining equipemt and is not capable to start mining from several different IPs?

[devphp]

No altcoin with POW is immune against that 51%. Not matter whether GPU or CPU mining. So POS coins could be an alternative.

Although it's true that PoS is much more protected, there is one PoW coin - Myriadcoin, that is technically considerably more secure than other PoW coins. Attacker would have to gain 51% hashpower in more than one (probably all 5) independant algos, otherwise, the network would still run without interruption.

[Lauda]

No altcoin with POW is immune against that 51%. Not matter whether GPU or CPU mining. So POS coins could be an alternative.

Correct. With POS coins the amount of $ needed to do a 51% attack goes up exponentially.

[justusranvier]

Here, what a user with 51% can do and not do:
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power

Mike Hearn is trying to enshrine the 51% attack into the protocol such that a majority cartel of miners can confiscate the generation transaction (just that, for now) of any other miner they want.

http://sourceforge.net/p/bitcoin/mailman/message/32258096/

[Peter R]

Here, what a user with 51% can do and not do:
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power

Mike Hearn is trying to enshrine the 51% attack into the protocol such that a majority cartel of miners can confiscate the generation transaction (just that, for now) of any other miner they want.

http://sourceforge.net/p/bitcoin/mailman/message/32258096/

Here are Mike's suggestions:

   1. Dishonest blocks can be identified out of band, by having honest
   miners submit double spends against themselves to the service anonymously
   using a separate tool. When their own double spend appears they know the
   block is bad.

   2. Miners can vote to reallocate the coinbase value of bad blocks before
   they mature. If a majority of blocks leading up to maturity vote for
   reallocation, the value goes into a pot that subsequent blocks are allowed
   to claim for themselves. Thus there is no risk to voting "no" on a block,
   the work done by the Finney attacker is not wasted, and users do not have
   to suffer through huge reorgs.

I dislike #2 because I see it as a slippery slope to coin confiscation, and I believe this requires changes to the source code.

#1 seems like something that can accomplished now without any changes to the bitcoin code.  Basically, it is a way for honest miners to come to consensus on which blocks were created by malicious double-spend services and agree not to build on top of them.  I think this idea may be worth evaluating in more detail.