A 51% attack on the network could reverse transactions.
As far as I understand how Bitcoin works this is false. What could happen is they could get unconfirmed as blocks in which they were confirmed become orphaned but they'd still be valid and pending for confirmation. This of course exposes some of those transaction to a double spend attack but it doesn't reverse them.
But how many of the hundreds of thousands of transactions now unconfirmed was the attacker a counterparty to? Likely very few, which I'm not trying to downplay as a weakness but saying that he can reverse transactions is just as inaccurate as saying it being impossible with mathematical certainty to cheat Bitcoin.
Btw mathematical certainty in my guide primarily refers to the ability to fake obeying the rules. It's true it doesn't account for an attacker obeying the rules and still defrauding people, but it's a fact that rules in Bitcoin must be obeyed, even by an attacker and this is mathematically certain.
Correct me if this is wrong, but if I had an extreme amount of computing power, I could create an alternate blockchain dating back to the genesis block, all the way to present, with the only confirmed transactions being me solving the blocks, and then alter the mining software with my new checkpoints. Also altering the mining software such that I ignore any information coming from the REAL miners with the CORRECT block chain. Then I become a miner with 51% of the network.
This would effectively REVERSE every transaction that ever occurred. Every existing transaction would appear to be an attempt to spend money that didn't belong to the owner, and so they would be reversed.
So although practically impossible, it is mathematically possible.
(I should also note that it would take just minutes to solve every block since the genesis block in an offline fashion, if I am creating this alternate blockchain, since I am not in competition with anyone, and the difficulty would be minimal.)
