1

Roughly, how many miners have to confirm the authenticity of a transaction in order for it to be added to the block chain?

2

Only one miner needs to verify a transaction for it to be added to the blockchain, but as additional miners mine blocks on top of the block that the transaction was included in, it becomes exponentially more certain that the transaction will be in the long-term chain.

There's no fixed number of confirmations that is generally accepted to be correct, and it depends on what is at stake if the transaction is dropped. I'd personally say that 100 confirmations is plenty for all but the highest-value transactions.

  • But then can a miner lie about a block being valid or not and still get added to the blockchain? This shouldn't be allowed because it would cause a hash mismatch, but if the miner has already added the transaction to the blockchain (it confirmed it) how can the network remove it? or would the block from the miner not be accepted in the first place? – Sebi Jun 23 '16 at 11:47
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    There is no canonical"blockchain" in the short term, each node maintains its own copy and consensus is only probabilistic in the long-run. The block will be added to the miner's blockchain, but other miners and nodes won't mine on top of the block, so it won't make it into the long term consensus. I think it may be valid as an uncle, since uncles don't affect state. – Tjaden Hess Jun 23 '16 at 11:51
  • The difference between an uncle block and a "normal" one is that the normal block has matching hashes for both data and headers while the uncle must only have valid header hashes, no? – Sebi Jun 23 '16 at 11:55
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    Yes, because the data in the uncle doesn't matter since it's never used. All that matters is the proof of work – Tjaden Hess Jun 23 '16 at 12:04

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