I keep hearing about a 51% attack but no time to read that long about it....
Any cliff note versions?
It would allow a "double-spend" attack.
That is, the attacker could buy something using bitcoins, and it would seem to the receiver that they had actually received the bitcoins (they would be confirmed on the blockchain), so they would ship the product.
Then using >50% of the network the attacker could create an alternate blockchain where that spend never happened. This new false blockchain would overtake the original blockchain because the false blockchain has more hashing power on it (i.e. >50% of the network). In this way the attacker can claw their bitcoins back, allowing them to spend the SAME bitcoins AGAIN with someone else. They could keep doing this until the world finds out about it (via news media), at which point everyone would panic-sell their Bitcoins until they had no value.
If this ever happens, no-one would trust Bitcoin or any Proof-of-Work cryptos (SHA256, Scrypt), since all PoW cryptos have this same vulnerability.
Basically it would be the end of Bitcoin. All our Bitcoins would be worthless (ditto for Litecoin, Dogecoin, Namecoin, etc).
Why would someone accept payment in Bitcoin if they knew it was possible that payment could be clawed back?
If we ensure that no-one has more than 50% of the global hash rate we KNOW a double-spend is not possible. If someone has >50% of the hash rate double-spends ARE POSSIBLE and therefore the integrity of Bitcoin is at stake.
See
http://codinginmysleep.com/bitcoin-attacks-in-plain-english/Note: >50% attack possibility is very real from the largest mining pool (currently GHash.IO).
tl;dr
The >50% attack would kill Bitcoin. No-one would trust Bitcoin, therefore no-one would use it.
LOTS of more mining power Im sure will hit the network and by July be a ridiculious number.
I dont see them hitting 51%, but then again Im probably wrong.
Let's hope the people taking delivery of those miners do the right thing for Bitcoin and don't point them at GHash.IO (or whatever the largest pool might be).
I think BTCGuild, Slush, and Eligius should all merge to compete.
Or bad idea??
We need to avoid having one dominant pool. A good selection of pools of roughly the same size would be best.
Ill stay with BTCGuild, theyve been stable so far, just shit luck lately
Over the long run it seems to get worse....
Great stuff! Thanks for doing your part in helping to secure Bitcoin.