Topic: If an attacker gets more than 50 % of mining power - page 2. (Read 7029 times)

Thiago, a related answer is here


Related thread:

Bitcoin's kryptonite: The 51% attack
http://forum.bitcoin.org/index.php?topic=12435.0

Sorry, I'm confusing, because I'm a newbie in Bitcoins world, just two weeks of knowledge about it...

Until today, I believed that miners only do mining... What they do more? Why miners are so important to keep the safety of the Bitcoins network?

If miners cash-out someday, this will leave the network unprotected.. But why?!?

By the way, then it is possible spend the same Bitcoin twice?! Since I have enough computing power for this?

And then it is also possible reverse the payment?!

All depends on the computational power?!

If the U.S. government and Fed tries to do these things today?! We are fucked?!

I'm very curious and concerned!

Thanks!
Thiago

I don't know.  I tend to be a firm believer in Emergent Order.  I usually assume that everyone is a self-interested bastard by default.  But Cooperation is more preferable to Sabotage in most cases, especially with bitcoin.  If the governments and banks cooperate with us, then they can profit from mining legitimately.  That's a positive sum game.  But spending billions of dollars on a ton of AMD GPUs simply so that you can destroy the bitcoin network is a negative sum game. 

There is a reason why successful governments like US & China don't tax %100 percent of your income, but rather realize that after about %50 then your people aren't incentivized.  And there are reasons why most Nations and Businesses are not engaging in constant battle sabotage against their competing Nations and Businesses (sure, there is some of this going on, but it is not the major component of expenditure).

I suspect that eventually the US government, PayPal, and banks will reluctantly embrace bitcoin.

Governments will still be able to tax, even if they can't arbitrarily inflate.  Sure they will have to tighten their fiscal responsibility belt, but they will be benefited by a robust bitcoin economy, and will simply tax visible transactions.  And I suspect they will be using the resources from government supercomputing labs to mine bitcoins by default when they aren't running more valuable experiments.

PayPal will start accepting payments with bitcoin as just one of many accepted international currencies the moment they realize that they can make more profit by embracing bitcoin instead of attacking it.  PayPal would rather offer secure online storage of your wallet and insurance against loss from scammer transactions as eBay currently does.  I can image some clever paypal employee submitting a bitcoin proposal which eventually reaches the CEO, who then sells it to the rest of the board and shareholders, and then someone might get a raise.

Banks wouldn't completely disappear, but would rather have to refocus on being investment vehicles which facilitate people with long term monetary preferences to loan out their bitcoins to borrowers with short term monetary preferences.  Sure they won't be able to artificially create bitcoins at whim out of thin air, but they would instead form clearing houses with competing banks and try to keep their liabilities as closely matched to their available assets as possible.  The banking sector will be stabilized and return to sanity, not destroyed.


Hmm... Yes I forgot about the lulz factor.  The lulz factor totally throws off anyone's refined security analysis model.  Serious, it's true.  Just look at everyone they hacked.  Totally not expecting it.  Totally were prepared for anything but a lulz attack.  (I'm only being 25% sarcastic those last 3 sentences, since it is true that the lulz factor will definitely screw up anyone who's not prepared for a lulz response.  Maybe it will become standard operating procedure for major internet companies to develop a "lulz response protocol" soon .)  But I do suspect that deep down, lulz security was motivated by something more than just the lulz...

If you have more than the rest of the network combined (so more than 50% of total power including you), you can grow the alternative chain indefinitely as long as you are ahead. If you have 1% more than the network, you will be 1.44 block ahead per day. If you have 10% more, you will have 14.4 blocks per day. If you have twice the current Bitcoin network, you will reverse 1000 blocks in less than a week.

If you want to do as much of a mess, grow your chain, and wait until the rest of the network caches up with the speed. And then release the alternative chain.

Not really. With 67%, you can reverse 1000 past blocks in a week. Reversing 1000 future blocks takes a week with 52%.

To reverse a 1000 blocks and to catch up then to make the longest block chain takes way more than 50 %, if you want to get it done in a decade.

For starters, he can create a new branch with none of the transactions from the last 1000 blocks. Or he can bypass X% of blocks as per your original post, which is not very dangerous for merchants but will piss off miners who wish to be rewarded for the blocks they find. And so on, he can get creative.

Assuming a consensus can be reached not only about which chain is the real one, but also that discarding the computationally longest chain in some circumstances is desirable.

Bitcoin never deletes blocks, so everyone will still have copies. The pools can be updated in no time, and they represent almost all of the network's hashing power.

You have to explain what you mean by 2.

• you don't understand, what the word "if" is for

I think Bitcoin can survive this happening once. The problem is if there is a reasonable expectation of it happening again. That's why the possibility of repeating it scares me.

If it happends once there will be no secont time bitcoin will be dead.

Personally I mostly fear the following scenario, which anyone can do at home (if he has a billion dollars lying around):
1. Short lots of bitcoins.
2. Build a huge mining cluster and completely mess up the block chain.
3. Watch prices drop as panic ensues.
4. Profit.
5. Lather, rinse, repeat.

It is not so hard for a rich guy or organization to spend 15millions to fuck bitcoin...

And what if the Earth explodes ?
And what if the aliens invade ?
And what if I am my own grandfather ?
And what if we're all living in a computer-generated virtual reality while being used as thermal-electrical generators (which would be extremely stupid because humans are extremely unefficient as powerplants but what if) ?
...
(continue as you wish into infinity, with absolutely no purpose)

Well he should buy the hardware and then run it long enough to have a longer blockchain

Once this happens how can you fix this? The only way would be to organzie a separate network with the old blockchain and telling miners to mine on it until it finally become once more longer than the "bad one"

It can be done but it require 1)a backup of the old blockchain 2)enough miners that know what to do

Yes, private attackers could be persuaded by the reward for honest mining. But banks and governments who are willing to invest to just shut down bitcoin are still a danger we should be aware of. I don't see any chance for them to get sabotage done, and it will get harder as bitcoin gets more users and miners. But we should always have the possibility of attackers in mind, who don't care about money but about hurting bitcoin.

Getting 50% would take a few million dollars, but rewriting many past blocks is much more expensive.

Even if any of this does happen, it can all be manually straightened out after control is regained.

You forget the "do it for the lulz" factor

But due to Sathoshi's genius clever design, would-be-attackers are instead incentivised to use their resources as legit miners instead, thus increasing the strength of the main block chain.