What is the real risk of a 51% attack?

[nameface]

I've heard that a network of miners large enough to represent 51% of the total hashing power of the entire network could push through false transactions and corrupt the blockchain.

Is this a real threat? Are there defenses against this?

If a rich private interest or government decided that this sort of attack was in their best interest, what's stopping them?

[Come-from-Beyond]

The blockchain won't be corrupted, just forked. And there are no viable defenses except trusted nodes concept.

[FreeMoney]

No one can ever 'push a false transaction' regardless of power, we all can and do instantly recognize and dismiss that. What a 50%+ attacker could do is undo some of their spends and re-spend them. Or they could refuse to put transactions in their blocks. Anyone can refuse to put any tx in their block, but it only matters much if someone is able to make all the blocks. 50%+ means you can make all the blocks because you can build your own longer chain.

The only 'defense' imo is to find automated ways to watch for hokey stuff like a fork from previous block and stop automated bitcoin systems (or maybe all) until you know wth is going on.

[justusranvier]

I've heard that a network of miners large enough to represent 51% of the total hashing power of the entire network could push through false transactions and corrupt the blockchain.

An attacker with suitable computing power could 1) prevent new transactions from being confirmed immedately. 2) rewrite history to delete transactions.

That being said the amount of computing power needed, especially for 2), is unlikely to ever be possessed by a single entity and getting less likely all the time.

[Stephen Gornick]

If a rich private interest or government decided that this sort of attack was in their best interest, what's stopping them?

Well, for beginners, what might be stopping them is a motive.  Why would this be in "their best interest"?

Another thing stopping them ... it wouldn't be easy, even for a nation state, to accomplish.  It takes more than an blank check to make this happen -- 25 Thash/s of hashing hardware is just not readily available.    Maybe when multiple ASIC vendors are duking it out there will be excess capacity available to this hypothetical attacker, but today it would be noticeable.

And if your government claims it has to cut back on school supplies for teachers, for instance, is spending tens of millions of dollars going after a cryptocurrency really the best use of funds which come only from current and future taxpayers?


But as far as technically being possible, yes, yes it is technically possible.  A lot of things are technically possible, but the reason they don't happen is there is no economic benefit to doing so.    As is the situation here.

[greyhawk]

If a rich private interest or government decided that this sort of attack was in their best interest, what's stopping them?

Another thing stopping them ... it wouldn't be easy, even for a nation state, to accomplish.  It takes more than an blank check to make this happen -- 25 Thash/s of hashing hardware is just not readily available.  

But it is. In a month. According to BFL at least.

We have currently activity of 21 Thash/s recorded at SIPA.

We have 22 confirmed preorders for 1 Thash/s BFL ASIC Minirigs on this board alone.

So should these things actually exist at one point - yeah, I know, it's funny but let's finish the thought - those 22 would represent more than all current hash power combined.

Now of course it's a given that these 22 would have to work together, but this is also doubly interesting. Because do we even now what exactly is running on these things?

[dissipate]

A first world government could launch a 51% attack on Bitcoin, but it would be quite pointless. The blockchain would just be reset back to before the attack happened and a patched client that rendered the 51% attack useless would be released.

[nameface]

A first world government could launch a 51% attack on Bitcoin, but it would be quite pointless. The blockchain would just be reset back to before the attack happened and a patched client that rendered the 51% attack useless would be released.

Interesting... This makes me feel all warm and fuzzy inside. Is there any hardcore assessment of the risks/defenses?

[mcdett]

Interesting... This makes me feel all warm and fuzzy inside. Is there any hardcore assessment of the risks/defenses?

I was in your shoes well over a year ago.  Spend some time here, and truly understand the roots of this.  An attack is possible, but improbable.  Everything in life has risks... even bitcoin, but don't take my word for it.  Come to your own conclusion.  There are no quick ways to put your mind at ease, just your own knowledge can do that.

[Etlase2]

A first world government could launch a 51% attack on Bitcoin, but it would be quite pointless. The blockchain would just be reset back to before the attack happened and a patched client that rendered the 51% attack useless would be released.

Interesting... This makes me feel all warm and fuzzy inside. Is there any hardcore assessment of the risks/defenses?

It shouldn't give you the warm fuzzies because it's inaccurate. If a government wants to take the chain down, they ignore all other miners' blocks and refuse all transactions. This attack can't be "patched out", a completely different metric other than hashing power for the block chain must be devised.

There's no real hardcore assessment. Anyone willing to devote a couple million could probably accomplish it.

[jgarzik]

Blockchain data is validated by every P2P node, miner or not.

Thus, even a miner with 100% network power cannot rewrite basic rules that other users disagree with.

[FreeMoney]

A first world government could launch a 51% attack on Bitcoin, but it would be quite pointless. The blockchain would just be reset back to before the attack happened and a patched client that rendered the 51% attack useless would be released.

This is wrong. A 'Bitcoin' that follows something besides the majority of the hashing power is not Bitcoin.

[dissipate]

A first world government could launch a 51% attack on Bitcoin, but it would be quite pointless. The blockchain would just be reset back to before the attack happened and a patched client that rendered the 51% attack useless would be released.

This is wrong. A 'Bitcoin' that follows something besides the majority of the hashing power is not Bitcoin.

Change the protocol, call it whatever you want. Bottom line is the government would be taking a big risk by trying a 51% attack. They would bring a massive amount of free press to Bitcoin, and ultimately fail to stop it.

[Desolator]

The value would instantly drop to $0.01 on every exchange as everyone jumps ship like the titanic so unless they're the first to put in a massive sell order (which is possible), they'd just kill the currency and make nothing off it.

By the way, it's not 51% as 50.01% would do it, as would 50.000001%.  The correct terminology would be >50% (greater than 50%)

At this moment in time, there are 2 things that could cause it.  First, there aren't enough GPUs to do it right now.  I mean there are but it'd take soooo many people/computers that it'd cost more money than they'd likely steal.

FPGA's are even more expensive in initial cost and I don't think the mine-able supply in the entire world would do it.

Possibility 1:
ASICs are fast as hell and nobody has them except one company that currently owns enough to beat the entire rest of the world by about 10-20%.  They'd need a hell of a lot of computers to run them and I don't think that that cost and effort = > $ than selling their products to people instead I don't think destroying BTC and thus their entire long term business plan is in their long term business plan either, lol.

Possibility 2:
That leaves a quantum computer.  There's like 1 working one or something and it can barely do math at the moment We're 5 years minimum from anyone producing one tailored to mining, complete with interface and programming instructions and everything.

[JoelKatz]

Change the protocol, call it whatever you want. Bottom line is the government would be taking a big risk by trying a 51% attack. They would bring a massive amount of free press to Bitcoin, and ultimately fail to stop it.

It would destroy Bitcoin. It would, however, be a huge boost to alternate crypto-currencies that don't use computing power to solve the double spend problem.

[Desolator]

Change the protocol, call it whatever you want. Bottom line is the government would be taking a big risk by trying a 51% attack. They would bring a massive amount of free press to Bitcoin, and ultimately fail to stop it.

It would destroy Bitcoin. It would, however, be a huge boost to alternate crypto-currencies that don't use computing power to solve the double spend problem.

Sounds neat.  How do they operate then?  Some kind of random number-based thing or something?

[dissipate]

Change the protocol, call it whatever you want. Bottom line is the government would be taking a big risk by trying a 51% attack. They would bring a massive amount of free press to Bitcoin, and ultimately fail to stop it.

It would destroy Bitcoin. It would, however, be a huge boost to alternate crypto-currencies that don't use computing power to solve the double spend problem.

Bitcoin Plus

[dissipate]

Change the protocol, call it whatever you want. Bottom line is the government would be taking a big risk by trying a 51% attack. They would bring a massive amount of free press to Bitcoin, and ultimately fail to stop it.

It would destroy Bitcoin. It would, however, be a huge boost to alternate crypto-currencies that don't use computing power to solve the double spend problem.

Sounds neat.  How do they operate then?  Some kind of random number-based thing or something?

Proof of Stake is one proposal for that.https://en.bitcoin.it/wiki/Proof_of_Stake

[Sage]

NSA will eventually use their computing power to attack Bitcoin.  You have to know this is a fata complete.  It's only a matter of time before Bitcoin is public enemy #1 to every parasitic central bank out there.  (And who do you think NSA takes their orders from?)

We really need to plan for such attacks and make the system robust enough to handle anything thrown at it.

[Melbustus]

NSA will eventually use their computing power to attack Bitcoin.  You have to know this is a fata complete.  It's only a matter of time before Bitcoin is public enemy #1 to every parasitic central bank out there.  (And how do you think NSA takes their orders from?)

We really need to plan for such attacks and make the system robust enough to handle anything thrown at it.

You forgot the </tinfoil>.


Government is not the efficient, scheming, effective, beast that you imply it is. Don't get me wrong - it is certainly a beast....just not one that operates anywhere near as well as would be required for the Federal Reserve to somehow direct the NSA to attack bitcoin. That's pretty laughable if you've ever spent time in/around government or the higher-ups in said agencies.


Edit: didn't mean to dismiss your greater point, though, that bitcoin should ideally be robust enough to thwart well-funded attacks...