Bitcoin Forum

So I was talking to a friend who introduced me to bitcoins and he was telling me about how a 51% attack would cause a lot of instability to the bitcoin network.

I still havent fully understood exactly how that would work.

First,

how do you gain control of 51% of the network? Is it by having 51% of the mining power or just own 51% of the bitcoins?

Also if someone go through with it, what would happen?

and my final question is;

Is there anything that can be done to avoid this in the future? Can the code of bitcoin be reprogrammed to not have such a weakness?(because it seems like a pretty big weakness to me)

This question has been asked and answered too many times to count..
Search this forum for "51%" and you'll find some very comprehensive answers.

It does not take if you have 0% - 1% or 90% of the bitcoins.
What matters is if you have more than 51% of the mining power. Miners are accepting transactions. It is a decentralized currency. If you have more than 51% of the power, you can freely choose which transactions can proceed or not. No matter if the owner of the bitcoins wants to send them or not. You can do whatever you want, because you are the majority. That's like if you got 51% of the voting power, you can choose the president, no matter what.


There is some option to reduce the risk of 51% attack, you can do some research about it.

I went and read some of them.

I guess what surprises me is that why is so much faith being put into bitcoin when an entity like the US government could easily attack bit coins at will.

Arent we being naive to believe noone will actually try and do it once they understand that its possible?

The US government obviously never wants the the Dollar to lose its status as the world reserve currency. Wouldnt they do anything in their power to prevent anyone else from contending against the USD? I realize bitcoin cannot contend at this point in time and probably never will.

However bitcoin still makes it easier for people to move money around anonymously. This is obv something that the US especially would NOT be very thrilled about.

ld love for BTC to succeed, but the 51% attack that apparently cannot be prevented seems like a very very big flaw, that might ultimately kill bitcoin.

Is it not possible to prevent this type of thing?

Don't we need an upgraded form of bit coin that isnt susceptible to this type of kill switch?

People have been well aware for a long time that a 51% attack is technically possible, however the bitcoin network has a huge amount of processing power, for example: http://www.geek.com/news/bitcoin-mining-network-8x-faster-than-top-500-super-computers-1554946/, although admittedly the hashes/second are not directly comparable to FLOPS, it gives some idea of how difficult it would be to get enough processing power to perform a 51% attack.

I'm pretty sure that if there was any obvious way to remove the risk of a 51% attack without sacrificing the advantages of bitcoin then someone would have implemented it. It's easy to complain about some theoretical weakness, but much harder to actually come up with a solution.

You really don't have a clue what is a 51% attack...

Actually you don't really have a clue what a 51% attack. What marcovaldo says is right.

Actually you just have to spend like 10 millions $ or so in ASIC to be able to successfully do a 51% attack.

::)

That is right, you choose wich transaction get confirmed and wich not. As you know an unconfirmed transaction is epic vulnerable to double spend and cannot be trusted.

Well yeah, if you can hash faster than the rest of the network you have control. Statistically, you should be able to create new blocks at a rate which guarantees that your chain is accepted above all others, i.e. everything else gets orphaned. If you wanted to stall the system, you could create blocks with no transactions and nobody would be able to send or receive coins.

My bad, I had something else in mind(mining on your side then releasing to the world). The attack you're talking about is so easily avoidable I keep forgetting about it.

If I understand them correctly, those two sentences are still wrong though:
You can't force people to send their coins
You can't change the protocol, nor steal coins for instance

umm what is a 51% attack?

Wrong, you can. You can rewrite all the blockchain from start if you wish. Then you will have mined every block. Thus, all btc will be your and of course the btc and all the transactions of other people will disappear. It is the same concept of a orphaned block, just much bigger. If you find a block and then it is orphaned because someone else found a block in the same second, you "lose" btc, right? Sure, you can say that "well, you never had them, the block was orphaned", but it was orphaned only after you found it.

http://lmgtfy.com/?q=51%25+attack

I'm not sure this is quite correct. My understanding is that this would require the attacker to produce a new blockchain that is bigger than the existing blockchain. That in turn would require not just having 51% of current mining power, but would require the attacker to do more hashing than the entire network has done in its history.  

Made my day :)

That is exactly the point of a 51% attack, having more hashing than the rest of the network. Consider that the old blocks were mined with GPU or CPU, it does not take a lot of ASIC to do more hashing than that. It is perfectly doable, if the attacker invest enough money and time (10 millions or so $)
Also it is not you have to rewrite the chain from 0, if you want you can start for example from a year ago

Ok this is stupid, the hash rate would skyrocket and while everyone else would make proportionally less lost to attack everyone would earn not as much for "whatever" and then they ask themselves, what are you doing? Even the attacker, but he would have to wait as long as it takes to clear out the rest of people that are not profiting and verifying books... mean time adding bandwidth to squash newcomers, with "2TH rigs by Jan." But 2T/H won't be close to enough by Jan.

Like any and all ventures in this nature, risk tolerance.

the perfect solution would be to have moral understanding that ASIC manufacturers wont send too many units out to one person....

..... life isnt perfect

A government/bank/whatelse can just start manufacturing their ASIC, it is not so hard for them.

The 51% Attack

What is it?

The 51% attack is perhaps the best-known of Bitcoin’s security vulnerabilities and it’s been on our collective radar since day 1. Bitcoin stores its transaction information in database chunks that are collectively referred to as the “blockchain.” As new transactions occur, they go into a new block on the end of the chain. While most of the time, the chain remains one straight line of blocks, on occasion a “fork” happens – two conflicting blocks appear and the network has to decide which one is valid. Each miner votes by continuing to tack new blocks onto the end of whichever block they think is valid – eventually the longest chain wins.

This also means that if I’m clever I can intentionally create two conflicting transactions, one in which I send money to a merchant and one in which I’m sending that same money to an address that I control. If I control more than half of the network’s processing power then I can decide which block ultimately gets accepted as true. Of course the longer back in time I go when creating these conflicting blocks, the more blocks I have to add to the end of my fraudulent chain before anyone accepts it as valid, so I’m pretty limited in how far back I can revise history and the further back I go, the more it costs me. This is the primary reasoning behind transactions not being considered complete until they’ve got 6+ confirmations.

Should I be worried?

For brevity’s sake: no, not really. The 51% attack is the oldest and best-understood attack in all of Bitcoin. Everyone is watching for it, we all know how it’s supposed to work and the “fix” is built right into the client – just wait for your 6 confirmations. Realistically, worrying about the 51% attack is a good indicator of high paranoia levels since it’s so amazingly cost-prohibitive to perform that we’re basically talking about a government focusing the full power of every top-secret ridiculously expensive supercomputer they’ve got at us – especially once some of those fancy new ASIC products start coming online.

-Source: http://codinginmysleep.com/bitcoin-attacks-in-plain-english/

government are not a threat. they know that 99% of the working population have employment contracts and live under the government rules of employment such as minimum wage in FIAT and tax in FIAT. so honestly, they dont care, bitcoin wont take over, not unless they change their own laws.... if anything it will just work alongside FIAT.

although this second it only requires 700T/hash to 51% (which if these ASIC manufacturers are quoting $15k/Thash) it would cost $10,500,000 to match the general public.

banks know that there are only 3600 coins produced a day which is less then half a million per day. banks make more money then this, trading on forex exchanges in just a few hours. both government and banks understand owning more then 51% makes things pretty much their property and under their control. but by doing so it loses the faith and the value of the general public. so there is no reason to 51% attack bitcoin,because it simply will de value their input.

secondly if governments were to make a reprogrammed client with new rules in their favour. the majority of us, would simply not download it. the same thing goes for them forking the chain during a 51%. the general public will just continue to run the previous versions of QT client and ignore the government version.

if banks were to stay under the threshold of 51% just to mine coins for their christmas bonuses. they would only get 1800 coins a day which equates to the low-mid $200k range per day. meaning it would take well over a month to break even. in that time the general public would have bought up units too making the hash rate increase further making the banks have to get more equipment just to keep a level income.

banks would prefer to spend $10million buying 80,000 coins and controlling the market, knowing if they only make 2.5% pumping and dumping the market daily they can be in instant profit, without spending money on equipments, technicians, storage warehouses, air cooling, electric, beyond the standard computers they use in their current offices.

and if you have ever watched MTGOX when just 1000 coins hits the order wall, then its easy to expect more then 2.5% swing in price using 80k coins.

so get mining out of your heads. banks and governments wont mine for profit or to attempt to ruin bitcoin. they will just buyout the exchange market for alot less money, hassle, risk.

Nonsense, if you manage to do a 51% attack, 6 "confirmations" are totally useless

Lol? Supercomputers? Why the hell someone should use supercomputers for the attack?

As i showed, it just take like 10 millions $ or so for the attack, so long for the "amazingly cost prohibitive" and the "focusing the full power of every top secret ridicolously expensive"  ::)