Quote from: AlexR12 on December 07, 2017, 07:00:59 PM

Quote from: mayo2u on December 07, 2017, 06:45:48 PM

One needs to find a correct hash to claim a block. Once you have done this work you need to do it again, again in order for your block to become the longest and for other miners to consider you branch to be the correct one. If you have half the hash power of the entire network you would expect to get every other block (over time) but, with half the hashpower. it is not inconceivable for you to get 5 blocks in a row (1:32) or even 10 in a row (1:1024).

Today, 2017, it's virtually impossible for one entity (not talking about a mining pool here) to get half the network hashrate. Still - there are scenarios where 51% attacks are possible.

My claim is stronger: with probability 1 a 51% attacker will be successful (as I see it, it follows from Satoshi's paper). What about the ordering question: What is the attacker's gain of changing the ordering of transactions?

Having 51% means that, over time, the attacker will be able to make the longest chain. In today's world that is highly unlikely. You would need to be getting 51% of the hashrate (highly unlikely) and not be detected as someone trying to reverse a transaction. Meaning that if one group of people are consistently adding work to a chain that is one behind the "real" chain - other miners will notice this and wonder why. As the attack continues they will look into the transaction details and find that the attacker is trying to reverse a transaction, then the attacker will be left all by himself. No one will be mining with him.

Again - for the attacker to succeed he needs to have more than 50% of the total hashpower. Why would he spend millions, if not tens of millions of dollars on a 51% attack in order to reverse a transaction? And which receiver of 100,000,000 dollars in bitcoin would not wait at least one full day (144 blocks) before releasing the product?

What ordering question are you referring to? The ordering within a block? Yes. Each miner will order the transactions in slightly differently.