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Comment by corimaith | Hacker News Reader

That's the point, you can only change YOUR history. From the perspective of future merchant, that's the trivial to deal with. And for existing transactions, you'd need the value of the goods from the transactions to exceed the cost of controlling to network to be worth it. But what kind of goods that can be transferred so quickly be worth that much?

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Maybe there's more resilience to prevent chain swaps now, but my understanding of the original blockchain algorithm is that:

At block N someone could start to privately mine (empty) blocks.

They keep mining in private until block N+x is public, at which time the private (51%) chain is length N+x+1.

They then announce their longer chain.

By the protocol, this longer chain (technically "most work" chain) is the more trusted one, and undoes any transactions in N+1 through N+x.

More or less, but the private chain doesn't need to contain empty blocks.

A more sophisticated attack would include all the legitimate transactions on the network except for their own transaction(s) which they're trying to double spend. That way the network isn't disrupted apart from the parties you're double spending against.

That way you can also claim 100% of mining rewards with 51% hash rate.

How? If that were true you’d also be able to get 50% of block chain rewards with 25.1% of the hashing power. But you can’t because it isn’t true.

You only mine blocks on top of your previous blocks, ignoring blocks produced by the 49%. Since you have 51%, your chain is the longest over time, so you have 100% of the mining rewards.

You can't do that with 25% (or even 40%) hashrate.

If you control 51% of the hashing power, that means you can solve more blocks than the entire rest of the network combined. Even if other nodes on the network solve a couple blocks before you, statistically, you will eventually create a longer chain of blocks and the network will switch to your chain.

But your chain has every block solved by you, giving you all the block rewards.

That's the magic of the 51% attack. You gain control of the blocks. Because that extra 1% isn't a HUGE margin, it may take a while for your chain to become the winning chain, but theoretically, it will happen.

Indeed, but I was arguing that the parent claim that "only your transactions" could be affected was false.

It's true that you can't synthesise false transactions, but you can undo anyone's transactions, not just your own.

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