Quote from: o_e_l_e_o on Today at 08:17:28 AM

There have been a number of occasions in the past where a single pool has controlled >51% of the hashrate, but they have not used this to attack bitcoin.

Deepbit in 2011 - https://bitcointalk.org/index.php?topic=26656
BTCGuild in 2013 - https://bitcointalk.org/index.php?topic=152296.0
GHash.IO in 2014 - https://bitcointalk.org/index.php?topic=645056.0
/snip/

Thanks for sharing. I thought ghash.io was the only time the pool went above 50%. I got into Bitcoin in late 2013 ao wasn't around when the first two times happened.

I guess we can draw the conclusion that the simplest and cheapest way to perform the attack is not to buy or produce enough hashpower, but to create a pool (or few pools pretending to be independent) and attract enough miners.

Quote from: o_e_l_e_o on Today at 06:22:25 PM

The 51% attacker can then mine an alternate chain to replace the last 20 blocks and include in that chain a transaction which sends the same 100 BTC back to one of their own addresses.

But that new chain would get rejected by the majority of the network, meaning nodes, right?