A recent comment from a Reddit user in response to BTC.top acquiring 54 of the Bitcoin Cash has hate noted that a Block Finalization development removes the threat of a 51% attack.

I have never heard of block finalization before but my understanding is that it results in nodes reject the altering of blocks once they have reached a certain amount of confirmation in the blockchain. At the moment, I believe it stands at ten confirmations in the Bitcoin Cash blockchain.

Is this a valid development? Are there limitations to it?

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    The fact that it's on r/btc should tell you enough: bull pushed by scammers. – Jannes May 21 '19 at 12:00

If ‘block finalization’ is needed, then your chain is broken. I say this because PoW is the method by which the Bitcoin network maintains consensus, and so the only reason we may introduce a ‘block finalization’ is if we worry that PoW will not be able to accomplish it’s job. If PoW cannot accomplish its job, then the system is already broken. It really is quite simple.

Consider: if 51% attacks are happening, and malicious miners are reorging the chain every few blocks, what good is ‘block finality’? The system is already seemingly broken.

Adding any sort of ‘block finalization’ can also introduce new issues and attacks:

Consider: if some nodes on the network somehow have a different history that goes back more than ten blocks, then the network will never be able to reach consensus without manual intervention.

So if a node is Sybil/eclipse attacked and fed a fake history by its peers, it will now never be able to get back onto the longest valid chain, because it will not follow a reorg of more than 10 blocks depth. In the face of a network at risk of 51% attacks, why should we assume that this would not happen? Rewinding 10 blocks could be quite easy for an attacker with a majority of hashpower, and so ‘block finalization’ just means the dangers associated with eclipse and Sybil attacks are far greater than before. Without block finalization, a node only needs one honest peer at any point in time, but with block finalization, a node needs one honest peer every ten blocks. A small but extremely important distinction.

Further, there are even situations in which this could cause issue without any malicious actors involved. If some part of the internet’s backbone infrastructure went down and isolated some portion of nodes and miners temporarily, then how would they ever get back in sync with the wider network? They wouldn’t be able to, without manual intervention.

So ‘block finality’ is not a technically sound idea. It is a bandaid for an already broken network.

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