What if a miner simply broadcasts the header and by no means broadcasts the physique?
The premise of this query is unsuitable: you can’t broadcast a header, as a result of the Bitcoin P2P protocol has no method of doing that.
It is doable to announce a header to a peer you are related to since BIP-130, nevertheless it’ll be interpreted as “and also you’re prepared to present me the block”, and absent the complete block information, it will not be relayed to their friends.
For instance: A miner, after having efficiently mined some block, simply broadcasts the header for all different miners and conceal the physique or by no means give the data of the physique. Beneath this case, what’s going to occur? Will all different miners simply begin the mining based mostly on that header?
Bitcoin Core won’t ever deal with a header with out block information as legitimate, nor will every other full node implementation I am conscious of. Miners will, someplace of their or the pool’s stack, want to make use of a full node to construct block templates with. As none of these will settle for only a header, it has no impression.
There does settle for a follow referred to as spy mining, which does roughly work such as you say, nevertheless it doesn’t contain block headers, or the P2P community in any respect.
The thought is that when a mining pool finds a block themselves, they will immediately change to constructing on high of it, as a result of they already know it is legitimate. But, the block will take a while to propagate by the community from there. Solely after propagation, validation, and constructing a brand new block template on high, can different miners change to constructing on high of that too.
With spy-mining, miners hook up with the pool interface of competing miners. Once they see, on this interface, a template for a block constructing on high of a block hash they don’t seem to be conscious of, they assume it is a legitimate block discovered by that pool, which hasn’t had the time but to totally propagate to them. Thus, they will change their {hardware} over to constructing on high of that block too. Since they do not know the contents of this presumed-valid block, they can not know what new transactions to incorporate, and thus they will create an empty block. It nonetheless pays them the subsidy (3.125 BTC as of writing) however lacks charges.
Nonetheless, that is harmful follow. In 2015, when BIP-66 activated, a non-upgraded miner discovered a block after activating (which was invalid by BIP-66’s guidelines). A majority of different miners, which have been upgraded, and thus would have been capable of acknowledge the block as invalid, have been participating in spy-mining, and began constructing empty blocks on high of this invalid block. Worse, they have been spy-mining on one another, and thus creating a series of invalid blocks one on high of the opposite, all empty.
This exhibits the chance right here: spy-mining, even a extra superior hypothetical kind that includes sharing full headers quite than simply hashes, solely works when the spied-upon miner is trusted to provide legitimate blocks, and be competent to not create accidentally-invalid blocks. In a sufficiently aggressive mining panorama, the place miners are nameless, and making an attempt to assault one another, it can’t work: miners can provide one another invalid blocks.
