Bitcoin Forum

As the title says, what will happen if someone did control more than 51% of the hashrate?

I've read about the possibility for double spending, but would it even be practical since it would be entirely based on chance, not hashrates?

If a single entity had control of more than 50% of the total h hashrate, they would be able to outvote the rest of the community. It's that simple. This entity would have the bitcoin network around his finger.

Some things like:

Preferential transactions
Illegitimate blocks
Blah blah blah end of the world etc.

Illegitimate blocks doesn't make sense (i.e. just because someone has the hashing majority doesn't mean that they can change consensus rules for others and in particular I am talking about a potential hard-fork).

The main concern would just be transactions being confirmed and then later becoming unconfirmed and replaced by others transactions through a re-org (i.e. done on purpose in order to "double spend").

It should also be pointed out that hashers could simply switch pools (as has occurred before) if they were concerned that the pool that they are using is getting "too powerful".

This happened once already with Btfury. They at one time had >%51 of the hash power. I believe what they did is split it up to a few nodes.
a 51% attack isn't in anyones best interest that uses BTC. so It's nteresting to see what will happen in the future if bitmain or bitfury get > 51%

he would create his own fork, and validate transaction or invalidate transaction of other, the legit chain would get invalid block also

Again - you can't create invalid blocks and they somehow become legit due to >50% of hashing power (that's not how Bitcoin works).

The rules about what is valid/standard is built into the software itself (so you need to change your software to change those rules).

(unfortunately ad-sig posters don't care about facts as they just want to post and get paid - so you constantly get nonsense posts being made on this forum)

When one single person has 51% of the bitcoins, then it would be possible for that person to create transactions that are invalid. However I think no one will ever own 51% of the bitcoins, unless a very rich guy turns all his money into bitcoin.

Again - *no* you can't create invalid txs or invalid blocks (what have you guys been reading to give you this idea?).

The issue is with potential re-orgs and not with validation.

The rules that apply to the blockchain is in accordance with the software that you are running and not to do with the POW determination of the longest chain (that applies *after* validation).

What are "re-orgs"?
I have been told that a 51% attack is very dangerous, what does it mean then?
I thought that double spend also was possible with 51% of the bitcoins.

Let's say that we have the following blocks:

A1 -> B1 -> C1 -> D1

but then someone with >50% of the hashing power creates this:

A2 -> B2 -> C2 -> D2

and the rest of the network agrees that the second set of blocks is both *valid* and *superior* (in terms of POW) than the first set.

So now A1..D1 are *discarded* and replaced with A2..D2 (what came before A1 and A2 is identical and therefore not of concern in terms of any change).

What this could mean is that txs that were valid (with 3 confirmations) in A1..D1 are now no longer valid after the re-org.

This is why you are recommended to wait for confirmations (the more value you are risking the more confirmations you should wait).

So the only thing that is possible is that a transaction can get canceled only when it has still no confirmation?

You didn't read very well as my example was 3 confirmations being *undone* (there is no real limit of rewinding beyond the difficulty to do this and the last "checkpoint").

So such attacks are very much *of concern* but I'm just pointing out that this concern is not about *invalid txs* or *invalid blocks*.

Okay, thanks for explaining.
I think the answer on the OP's his question has been answered, so the thread can be closed. Thanks for clearing things up for me, now I understand it a little better.

wiki "Prevent some or all other miners from mining any valid blocks", the same thing i've said in other words

That does not imply "mining invalid blocks" or are you so stupid that you can't understand the difference?

(my guess is the latter)

my guess is that you are not intelligent enough that you didn't udnerstand what i was implying

if none of them will mine valid block then they can be seen as invalid

Huh?

Moron?

Care to try and explain yourself in English?

(you think that if no-one validates a block then invalid blocks get mined?)

no, in the sense that they can not be validated by the miners on the orignal fork, not that they mine block that are seen as invalid by the netowork

invalid blocks = block that miners on the original fork can not validate anymore

i like how you try so hard about semantic just to have reason on an argument, but it simply misunderstanding as usual

The beautiful part for me in Bitcoin is this.... if you planned to build up enough hash rate to hurt Bitcoin, you would have to consider this :

1. It's going to cost you a shitload of money. { If you go solo, without other pool members }

2. It would be like cutting off your nose to spite your face. { Imagine what will happen if you did manage to do this and you make changes nobody else wants? You would be killing the cow, that
    produce the milk. Everyone will start selling to bail out, and you will be left with a empty bag, or a bag full of worthless coins. }

It is in everyone's best interest to keep Bitcoin properly decentralized and to make sure the code that are being accepted, is something the majority wants.. otherwise you would be the only one in

the sandpit, playing alone.  ;)

Which makes absolutely zero sense (like most of your posts so no real surprise there).

*any word* = *whatever you want to say it means* is not a valid way to answer a question. :D

you can give different sense to some meaning, it's happen all the time, it was a faster way for me to say the same thing on the wiki

this does not imply that i didn't understand the concept

So CIYAM, I know that it would be literally completely unprofitable for any person to launch the 51% attack. Basically bitcoin will go down in price, and the guy who launched the attack won't be able to sell those coins for a profit.

But what if, say an organization or a government decided to do this, just because they hate BTC? They've got tons of processing power to waste, and currency to invest. What are the chances of that happening?

Should we be worried at all, or do you think that bitcoin is going to work as usual in the future?

I would guess as the majority of hash power is located in China that its government could be the biggest such potential threat, however, unlike Russia we have not seen any negative reports about Bitcoin in the Chinese media so for now I think Bitcoin is looking fairly safe from a >50% attack.

Certainly spreading the hash rate across more countries would be a good thing to strengthen Bitcoin from any such potential threat (and mining farms in places like Iceland are already helping with this).

Yes, China is a very big threat IMO. Currently bitcoin is used as a gateway to get CNY out of the country. They do have a motive to stop that.

There are a couple of pools that have came close, but they have been legitimate enough to not launch an attack.

Can the bitcoin devs do anything about it, or is there a good solution to a problem?

There was a list put together of ten ways that Chinese use to move RMB (CNY) out of the country and Bitcoin was at the very bottom (accounting for considerably less than even 1% of said currency movement) so I don't think it is going to be focused on for that reason (any time soon at least).


As long as mining farms are set up in as many different places as they can be then Bitcoin will be fine (there isn't really anything much that can be coded to help this).

People get worried when they hear 51 percent.   Ghash pool got it.... then fell drastically, people switched to make sure it did not stay that way.  So we have a example where a pool did achieve 51 percent.  And no bad things happened.

For a pool or person in bitcoin to perform any 51 percent attack would tank BTC price.  So in most if not all cases hurt their business.  So I don't think we will ever see it happen on an attack between it not being in best interest and people switch pools if it gets that high.

As I was suggesting what if a government/organization just want to destroy the bitcoin network? Not for profit. Rather for political or economic purposes.


According to one thread though, China is currently controlling 80% of the hashrate. Would that become a  worry if they decide to join together?

Do you think a 51% will happen at all?

A worst case scenario would be the forced takeover of mining farms by the Chinese state. Of course if such a thing were to occur then the price of BTC would plummet (so such a move would be politically rather than economically driven).

It is hard to see any way around Chinese domination in one way or another as the vast majority of computing devices in the world are manufactured in China (and western corporations can only blame their own greed for creating this situation in the first place).

Even if you had a coin that could be mined with smart phones (i.e. requiring very little electricity) in all likelihood China would control the majority of such nodes as well (as nearly every Chinese that I see on a daily basis has a smart phone - and I mean even cleaners and construction workers).

In some ways perhaps it could make sense that more localised block-chains might serve a useful purpose (although one of Bitcoin's biggest selling points is it being global and not under the whim of any government).

Also in places such as Iceland where electricity is virtually free and temperatures are low enough to require little (or maybe even no) cooling there will always be a potential for growth so we might find that hashing power ends up concentrated in such locations over time (I guess then it is a matter of how many such locations there are and in how many different countries).

Mostly this last one, coz they would not risk devaluate bitcoin, but better noone has 51%.

They probably will get most of the fees in a block.