What will happen if someone did control more than 51% of the hashrate?

[Cyaren]

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?

[Decoded]

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.

[CIYAM]

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".

[MRKLYE]

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?

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%

[Amph]

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

[CIYAM]

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)

[Bettwarm]

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.

[CIYAM]

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).

[Bettwarm]

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.

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.

[CIYAM]

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).

[Bettwarm]

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 that 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?

[CIYAM]

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*.

[Bettwarm]

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.

[Amph]

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)

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

[CIYAM]

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)

[Amph]

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

[CIYAM]

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?)

[Amph]

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

[Kprawn]

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. 

[CIYAM]

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

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.