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As per my understanding of the blockchain so far (Bitcoin and Ethereum) nodes and miners have different jobs to do in a blockchain network. Although the terms are extensively used interchangeably, nodes are smaller and more simpler computers which only act as transaction validators and stores a copy of the blockchain each whereas miners are the complex and expensive ones which does all the computation and mine a token of the blockchain and have to keep the proof of work. Now my question is why it's said that theoretically to conduct a successful Sybil attack, one has to gain control of over 51% of the computation power (which of course would be extremely expensive and impractical to do) and not gain control of majority of the nodes (which would be fairly practical to do as compared to the prior) which would give the attacker validation even for the malicious transactions as now he simply would have the majority voting rights?

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Because there is no majority vote in Bitcoin and Ethereum - there is a rule of choosing which of the competing branches of blocks will be joined to the main chain at the nodes of the network. And this rule is the support of the longest of the branches. And for the stable formation of the longest branch, you need to have 51% of the computing power.

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  • But nodes are the ones who first cross checks every transaction completed and included in the block by the miner and provide "legit" validation for the block to be added in the blockchain isn't it? Dec 5, 2021 at 17:41
  • Note that this only applies to proof-of-work, not proof-of-stake, which has a bit different consensus mechanism. Dec 5, 2021 at 17:47
  • @Manav Kampani > In Bitcoin and Ethereum, nodes are independent of each other when building a chain and cannot influence other nodes except for the proposal of their candidate block for inclusion in the chain. If the block is formed incorrectly, then it simply will not be accepted by any node.
    – Mad Jackal
    Dec 5, 2021 at 17:48
  • @MadJackal that is what I am trying to ask that what if the attacking organization owns majority of the nodes maliciously programmed to accept blocks from their miners? Dec 5, 2021 at 18:02
  • The attacker's task is to have his branch of the chain accepted by neutral nodes. And the number of nodes of the attacker has no effect here, since two rules apply to neutral nodes when choosing a chain: 1. the blocks must be valid, 2. The branch must be longer than the competing one
    – Mad Jackal
    Dec 5, 2021 at 19:57

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