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How about a scenario where a few miners change their protocols by setting difficulty to something easy. Let's take it to the extreme and say they are able to accept any block.

They would keep publishing blocks which they think are valid and those would get rejected by the network. What if more miners start joining that scheme though (by changing their protocols)?

Because they are able to publish blocks extremely quickly (because they set their difficulty low) they might actually be able to compete with the whole network and their branch might overtake the network branch even if they are not that many of them.

What am I missing here?

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Contrary to popular belief, the longer blockchain is not the one with the most blocks. Rather it is the blockchain which has the most proof of work. Each block has a field for indicating the difficulty (i.e. work) for that block. The longest blockchain is the one that has the largest sum of these difficulty fields. The only way for a fork to overtake the current blockchain and replace it with the fork is if the miners on the fork have equal to or greater hashrate than the miners on the current blockchain. Otherwise the miners on that fork won't be doing as much work as the miners on the current blockchain so they can never overtake the blockchain.

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  • That is a very good point, thanks. How about other changes to the protocol? For example something minor. Is it possible that most miners have made some changes to it but the network simply doesn't know because from their point of view everything is normal? In other words, is there any consensus mechanism for the protocol? (I haven't come across anything like that reading around)
    – tsotsi
    Jun 18, 2017 at 22:14
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    What you are describing here is a soft fork. A soft fork is something that changes the consensus rules without breaking backwards compatibility. These forks only require miners to upgrade, whereas any other consensus change requires everyone (nodes, wallets, etc.) to upgrade. However soft forks are limited in what they can do as they must be backwards compatible. They can only make something that was previously valid, invalid.
    – Andrew Chow ♦
    Jun 18, 2017 at 22:18
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    @tsotsi If miners produce blocks that violate consensus rules, their blocks are ignored by nodes. Is there a consensus mechanism for the protocol: yes, politics.
    – Pieter Wuille
    Jun 18, 2017 at 22:19
  • @AndrewChow Is it a correct statement to make "the longest chain(with the most blocks) must have the highest PoW backing it" because other forks violating the current difficulty parameters will not be compatible with the existing chain
    – sanket1729
    Jun 19, 2017 at 5:47
  • @Sanket Kanjalkar Yes
    – Andrew Chow ♦
    Jun 19, 2017 at 6:34
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The rest of the network, which uses the existing difficulty parameters, will reject the new blocks generated by your modified miners. There will be a longer blockchain, but there will not be a longer valid blockchain according to the established rules.

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  • As soon as they use an incorrect difficulty on even a single block, nobody else will accept that block or any block after it ... ever.
    – David Schwartz
    Jun 24, 2017 at 11:41

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