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Let's say someone creates a fraudulent block and finds a nonce that solves the hash puzzle. This block is broadcasted, its hash easily verified, and it's added to the blockchain. What prevents a miner from hearing this block and adding it to its blockchain such that it uses the hash of this fraudulent block as the previous hash field in the new block it is trying to solve? Couldn't this then create the longest blockchain at a fork and thus become accepted?

When do mining nodes give up on a certain block? Once a block with the correct nonce is found for a given previous block, don't all other nodes mining a block with the same previous node have to start over?

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  • What do you mean by a fraudulent block?
    – Matthew Stannard
    Jan 7, 2018 at 7:41
  • A block that contains an incorrect or malicious transaction.
    – flyinghigh
    Jan 7, 2018 at 7:43

2 Answers 2

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A miner could create a block with an incorrect transaction, but this wouldn't be accepted by the honest nodes in the network. The honest nodes would attempt to validate the transaction, fail and then not propagate the block to other nodes.

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  • What is an example of someone trying to modify a transaction (and then embarking on the impossible task of redoing PoW for every subsequent node). In this case, the change is something validated and thus the need for PoW?
    – flyinghigh
    Jan 7, 2018 at 7:58
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    PoW does not really matter here. Blocks are verified before PoW even comes into play. If an invalid block is produced, nodes simply ignore it. Then, PoW is used to find the most-work chain out of the set of valid blocks.
    – Pieter Wuille
    Jan 7, 2018 at 9:05
  • Then what is an example of a valid block that is trying to overtake a legitimate blockchain, but won't be added because it would have to redo immense amounts of PoW.
    – flyinghigh
    Jan 7, 2018 at 21:19
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Every full node independently validates that each block is following all consensus rules of Bitcoin. Therefore, a block with invalid transactions would simply not be accepted by any full node, and thus would quickly be forgotten by the whole network.

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