Bitcoin Forum

So what's the deal with people talking about 50%ing a pool? What happens if you take over 50% of a pool's hash rate? Does it start causing a lot of stales for other users, or what? Just curious. I've never been in a position to be of any risk of doing that, EVER, so it's just a curiosity thing.

not sure about 50%ing a pool, but maybe you mean

https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power

and a lead developers input
http://gavintech.blogspot.com.au/2012/05/neutralizing-51-attack.html

Having 51% or more of the bitcoin hashing power at your disposal opens up a few interesting possibilities.  However, luck is still a factor.  The more hashing power over 51% you have, the easier it is to do bad things.  51% hash power means in the statistical long run you can generate a longer block chain than everyone else and can therefore control what goes in it.

Some possibilities..
- you could blacklist another pool's blocks, forcing them to be orphaned and drive them out of business.
- you could collude with people to facilitate double spends by orphaning blocks containing "confirmations".

Reaching 51% doesn't make the world end, and there's still a luck component.  The pool operator also has to have nefarious intentions and that seems highly unlikely in this case.

The real issue is going from *knowing* somebody can't do this sort of thing to having to *hope* that they don't.

tl;dr
Think of BTC miners as a jury pool. If a single entity controls 50% of the pool, they can sway the jury for bad things.
It's better for everyone involved if the 50% mark never comes to pass.

do note that even if there is someone managing to reach > 50%, they still cannot just do anything.  You can ignore certain transactions, make sure they are not inside a block, therefore disrupting bitcoin, but there are limits to what you can do, and you certainly cannot just create  money or steal from someone else.

Also note that even if a pool reaches >50%, they still need to abuse their power, which will be detected and people will be warned... it's not something you can do in secret.  Also note that >50% gives you a theoretical advantage, if you want to abuse your power,you can do so also with for example 10%, you will just have a lower successrate.

However, if a pool is really abusive and has 50% of the hashrate, one cannot stop them due to lack of power.   So yes, it is good to make sure nobody ever reaches 50%. 

There are many good smaller pools out there, if you want to support bitcoin, make sure you get to a smaller pool :)

So basically having over 50% doesn't mean the sky is falling.  The person (or pool operator) has to want to do bad things.  If he/she/pool just keeps mining as usual nothing bad can happen really?

Correct.  50%+ doesn't give anybody magical powers.  They can't change the core rules of Bitcoin, they can't take other people's money.  It simply opens up two major attack vectors:

1) They can deny all other blocks except their own since they will always eventually produce the longest block chain.
2) Double spending becomes extremely easy/almost guaranteed to be successful due to #1.


Both of these cannot be done in stealth, there would be signs of something happening.  However, just the fact that it COULD happen if an entity went rogue is a bad situation, and would be harmful to Bitcoin's reputation, even if it never took place (there would be no economic incentive for a pool charging reasonable fees to do something that would kill Bitcoin).  Unfortunately, the problem also shifts to:  Well what if somebody compromised the 50%+ pool and did something evil with that power without the owner's knowledge.

So that's why the 50% thing isn't that bad for pools.  If one pool would have more then 50% and if they would abuse their power, there will be a lot of voices on the internet pointing in the direction of that pool and saying "they're attacking bitcoin".  At that point, hopefully enough miners would get the message and switch away to another pool.

The real danger is if a single entity would obtain 50%, capable of mining itself with no help from others.  Not a pool, a pool manages the hashing capacity of it's users, and the users make the decission to mine with that pool, so if the miners trust is abused, they can move away...

In any case, prevention is the best solution, so the best thing to do is to just stay away from the pools with  more then 10-20% of the hashrate...  Give the smaller pools a chance to compete...