Why is 51% the magic number for attacks?

[lynn_402]

Wouldn't someone be able to attack the network with only 40% of the hashpower, and he'd only need more luck?

And why do everyone seem to assume that as soon as anyone have 51% of the hashpower, he can do whatever he want; wouldn't he still only have about 2% chance of solving 6 blocks in a row?

[rigel]

Wouldn't someone be able to attack the network with only 40% of the hashpower, and he'd only need more luck?

And why do everyone seem to assume that as soon as anyone have 51% of the hashpower, he can do whatever he want; wouldn't he still only have about 2% chance of solving 6 blocks in a row?

The magic is in more than half.
51% is not precise: this should be "more than 50%"

[lynn_402]

Wouldn't someone be able to attack the network with only 40% of the hashpower, and he'd only need more luck?

And why do everyone seem to assume that as soon as anyone have 51% of the hashpower, he can do whatever he want; wouldn't he still only have about 2% chance of solving 6 blocks in a row?

The magic is in more than half.
51% is not precise: this should be "more than 50%"

But isn't 48% almost as dangerous? When ghash was bigger a few weeks ago, it seems like people were considering 50% to be the critical point that needed to be reached before they did anything bad.

[virtualx]

Wouldn't someone be able to attack the network with only 40% of the hashpower, and he'd only need more luck?

And why do everyone seem to assume that as soon as anyone have 51% of the hashpower, he can do whatever he want; wouldn't he still only have about 2% chance of solving 6 blocks in a row?

The magic is in more than half.
51% is not precise: this should be "more than 50%"

But isn't 48% almost as dangerous? When ghash was bigger a few weeks ago, it seems like people were considering 50% to be the critical point that needed to be reached before they did anything bad.

48% is a lot, but it's not more than half. It's not likely that something would happen, as miners want to gain the most profits - so they want bitcoin stays alive.

[lynn_402]

Wouldn't someone be able to attack the network with only 40% of the hashpower, and he'd only need more luck?

And why do everyone seem to assume that as soon as anyone have 51% of the hashpower, he can do whatever he want; wouldn't he still only have about 2% chance of solving 6 blocks in a row?

The magic is in more than half.
51% is not precise: this should be "more than 50%"

But isn't 48% almost as dangerous? When ghash was bigger a few weeks ago, it seems like people were considering 50% to be the critical point that needed to be reached before they did anything bad.

48% is a lot, but it's not more than half. It's not likely that something would happen, as miners want to gain the most profits - so they want bitcoin stays alive.

Sure, but my question was; is there anything that someone with 51% of the network can do that someone with 49% of it can't?

[virtualx]

Wouldn't someone be able to attack the network with only 40% of the hashpower, and he'd only need more luck?

And why do everyone seem to assume that as soon as anyone have 51% of the hashpower, he can do whatever he want; wouldn't he still only have about 2% chance of solving 6 blocks in a row?

The magic is in more than half.
51% is not precise: this should be "more than 50%"

But isn't 48% almost as dangerous? When ghash was bigger a few weeks ago, it seems like people were considering 50% to be the critical point that needed to be reached before they did anything bad.

48% is a lot, but it's not more than half. It's not likely that something would happen, as miners want to gain the most profits - so they want bitcoin stays alive.

Sure, but my question was; is there anything that someone with 51% of the network can do that someone with 49% of it can't?

As far as I understand it, together they could manipulate transactions - such as make transactions that did not occur.  However, if they would do this - the market price would quickly drop.

[azeteki]

Sure, but my question was; is there anything that someone with 51% of the network can do that someone with 49% of it can't?

Any miner can attempt to produce a private chain, and release it if it outpaces the network in work done. This chain would then displace the main chain. Hashrate increases the probability, but there is a discontinuity above 50% (because you become the majority).

Imagine a network with a total of 100 hashes per second. The real network is not actually constant hashrate, but it's easier to explain with figures.

With 48% of that network, you produce 48H/s, while the rest of the network produces 52H/s.

A luck streak could mean that you produce blocks faster than everyone else for a bit. It's random, after all. You won't know in advance that you will win, but you have a reasonably high chance. The honest network mines on average about 8% faster than you. (52/40)*100 ~= 1.08.

With 52%, you have 52H/s while everyone else has 48H/s. You mine on average 8% faster than them.
You produce blocks faster on average. It might take a day, or a week, or a month, for statistical fluctuations to average out; but eventually you win (so long as you can maintain 52%).
And so you can mine every future block. Just ignore anyone else's blocks but your own, and only mine on yours.

[Simon8x]

Wouldn't someone be able to attack the network with only 40% of the hashpower, and he'd only need more luck?

And why do everyone seem to assume that as soon as anyone have 51% of the hashpower, he can do whatever he want; wouldn't he still only have about 2% chance of solving 6 blocks in a row?

The magic is in more than half.
51% is not precise: this should be "more than 50%"

But isn't 48% almost as dangerous? When ghash was bigger a few weeks ago, it seems like people were considering 50% to be the critical point that needed to be reached before they did anything bad.

48% is a lot, but it's not more than half. It's not likely that something would happen, as miners want to gain the most profits - so they want bitcoin stays alive.

The following bitcoin wiki page should be helpful to you.

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

With less than 50%, the same kind of attacks are possible, but with less than 100% rate of success. For example, someone with only 40% of the network computing power can overcome a 6-deep confirmed transaction with a 50% success rate.

[minerpumpkin]

Sure, but my question was; is there anything that someone with 51% of the network can do that someone with 49% of it can't?

As far as I understand it, together they could manipulate transactions - such as make transactions that did not occur.  However, if they would do this - the market price would quickly drop.

Thing is: They could deposit BTC on some exchange and trade it for another coin or possibly FIAT and then - with 51% - secretly mine a chain that's longer than the regular blockchain. Due to the law of big numbers, they will have a longer chain per definition if they're above 50%. They now can double spend those BTC they're traded for other coins before. Profit.