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I'm pretty sure that I understand how replay attacks work: A malicious node takes a valid transaction from chain A, and replays it on chain B. Other nodes on chain B see the transaction as valid and therefore it is included in both chains, instead of just A.

My question is whether the first transaction in a block, that generates new bitcoins, can be also replayed.

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    Are you literally asking if a coinbase transaction from one chain can be used as a coinbase in a block on the other chain? Under certain assumptions, yes, but it's pointless to do so: if you mine a block using a "replayed" coinbase you are just donating your block reward to the previous miner. It hurts the "attacker" and benefits the "victim". – Nate Eldredge Oct 15 '17 at 20:00
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Short answer: No.

Longer answer: depends on the rules of the B chain.

A coinbase transaction creates coins out of thin air, but is typically only valid if the block itself is valid. Typically if the block is valid on chain A it's not valid on chain B: because a block usually points to it's previous block, which should be on the same chain.

On the other hand, the rules of B chain can literally be anything, as crazy as the creators imagination allows. If the B chain is specifically designed to also allow A blocks, then the answer would be yes. But the coins would still belong to the miner, so it's hardly an attack.

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  • So what makes coinbase transactions impossible to replay, as opposed to regular tx's, is that they come together with a block, and that block is typically illegal on chain B. – Nathan_ Oct 15 '17 at 19:58
  • Yes. And if it were accepted on chain B, that money is still owned by the miner who has the private key to that coinbase address. Hardly an attack. – Jannes Oct 16 '17 at 3:11

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