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I am new to blockchain and quite curious about some attack in bitcoin network. So, assume there is already an existing block which has content A and address B. Let's say if an attacker changes the block's content to A' and changes the address to B'. Just curious what should the attacker do to get the reward form the current? Does he need to manually create a hash collision? Or he may need to steal the pub key and signature?

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From Bitcoin's actual rules, you would have to do several things:

  • Solve the nonce for that block (simple mining problem) with a fake timestamp.
  • Intelligently calculate the rest of the nonces until you have enough chainwork to overtake the current canonical chain, without the timestamping going into the future (a byproduct of attempts to artificially lower the 2048 block difficulty calculation)
  • Successfully partition enough of the network to cause a reorg that causes the consensus to latch onto your fork. This requires several eclipse attacks.

Each one of these steps' difficulty increases astronomically, but is still technically possible.