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What if I deployed 3 of my personal tampered/hacked full nodes(with one of them a miner too) that accepts invalid blocks with proper PoW (containing invalid transactions) and rejects valid blocks. Assume there were previously 2 neutral full nodes in the blockchain with one of them a miner. Whose copy of the blockchain will be the true one now in such a case where we have majority of tampered full nodes accepting invalid blocks.

Also is this what's called a sybil attack in a blockchain?

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The other two nodes would attempt to validate the invalid block, find it to contain invalid transactions, and thus those nodes will discard and ignore it.

Those two nodes will continue to follow the longest chain that is valid according to the rules they enforce. There could be one million other nodes that all follow the invalid block, but that would be without consequence to the two ('honest') network peers.

Whose copy of the blockchain will be the true one now in such a case where we have majority of tampered full nodes accepting invalid blocks

If they continue to follow the invalid chain, then the malicious nodes would create a new network, due to an incompatible (hard forking) rule change. The original network would continue to operate, otherwise unaffected by this.

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  • is this called a Sybil attack or a 51% attack? (my guess is it cannot be called a 51% attack because we don't have the majority hash rate but only influence majority of full nodes)
    – Manav Kampani
    Commented Dec 7, 2021 at 10:13
  • @ManavKampani A majority attack (51% attack) involves controlling a majority of the hashrate. What you've described isn't really a sybil attack either, as the dishonest nodes will just be ignored/banned by other nodes if they continuously misbehave. What you're imagining is perhaps most similar to an eclipse attack (which can overcome by making a connection to only one single honest peer).
    – chytrik
    Commented Dec 7, 2021 at 21:03
  • so basically even if there's a single working neutral node in the network (assuming it's also a miner node) the blockchain will work just fine and secure.
    – Manav Kampani
    Commented Dec 8, 2021 at 5:52
  • @ManavKampani well, if there were zero honest nodes, then there wouldn't really be a network to speak of in the first place. No number of dishonest nodes (or miners) can convince an honest node to accept an invalid block. For eclipse attacks, see: bitcoin.stackexchange.com/questions/tagged/…
    – chytrik
    Commented Dec 8, 2021 at 8:15
  • okay but I'm asking about a single honest full miner node, will the network work then?
    – Manav Kampani
    Commented Dec 8, 2021 at 8:21

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