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I know that ASIC BOOST exploits the fact that the blockheaders are not a multiple of 64 bytes, but what is the change in Segwit that prevents this? Are they always a multiple of 64 bytes now for example?

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There's 2 versions of ASICBOOST:

  • Overt where miners use bits in the version number as extra nonce space
  • Covert where miners "mine" merkle trees with 4 bytes collisions

The overt version is very easily detectable, whereas the covert one isn't.

To mine these merkle trees for the overt version, miners need to shuffle the transactions in the block.

Without Segwit transactions in a block, it is possible to do it in a very low number of SHA-256 operations ; with Segwit transactions, a change of order forces miners to recompute the witness commitment which is stored in the coinbase. That makes it way more expensive to mine merkle trees and destroys the advantage covert ASICBOOST provides.

You can find a detailed explanation here.

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  • SegWit doesn’t eliminate the ability to achieve the full performance of covert ASICBOOST. Study more carefully the source you cited. If the block has only 1 or 2 transactions, then the difference in covert ASICBOOST performance is not very significant. Given Lightning Networks, we can expect the possibility of settlement transactions that can occupy an entire block. So then even blocks with only 1 or 2 transactions will not be conspicuous evidence of covert ASICBOOST. Let’s presume LN Mt.Box hubs can possibly collude with covert ASICBOOST miners.
    – Shelby Moore III
    Commented Jun 13, 2018 at 19:18
  • It is true that the less transactions, the overhead is lessened. However, Lightning settlement transactions don't take more space than other transactions. The coooperative case is a simple 2-of-2 multisig spend, and thousands of those happen per day already.
    – alcio
    Commented Jun 14, 2018 at 10:06
  • Bitcoin will become a settlement network between whales with huge transactions with a plurality of inputs and/or outputs. Think of centralized exchanges which is what LN Mt. Box hubs will become. The on-chain transaction fees with be $50,000. Only the uber wealthy will transact on-chain and open LN channels on-chain. Everyone else will be on fractional reserve banking. That is the master plan and the only way the technology works.
    – Shelby Moore III
    Commented Jun 15, 2018 at 23:46

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