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Andreas Antonopoulos, the author of Mastering Bitcoin and well-known Bitcoin proponent, spoke about unknown block versions being mined in relation to the Bitcoin blockchain, during the latest Q&A session on YouTube. The author was asked whether this meant that there were possible unknown rules in effect as a warning message stated, ‘Unknown block versions being mined as possible unknown rules are in effect’ pops up when one runs a Bitcoin core node.
Antonopoulos stated that the message meant that miners were using the block version field with the block header in order to create more possible permutations of the block hash. He stated that the “most likely use” of this was to take advantage of ASIC boost, adding that there were different ways to take advantage of ASIC boost.
Here, ASIC boost refers to a shortcut in the SHA-256 algorithm, that enables miners to mine with around 20 percent less energy for the same number of hashes. The author stated that ASIC boost was used by several miners in order to reduce their energy costs, adding that it was a “competitive advantage”. He further stated that it could “significantly affect the profitability of a miner”. He went on to state,
“[…] when ASIC boost is being used, because of the particular structure of the block header and the way this shortcuts in SHA-256 works, they need to change some bits in some part of the header in order to take advantage of ASIC boost. Given that there are not necessarily that many places where you can make changes in the block header without it affecting the outcome, they’ll use the block version […]”
He further stated,
“[…] and your Bitcoin node will complain about this because Bitcoin nodes are using the block version to try to figure out if the consensus rules have changed and this is from a time when the block version was used to signal various votes in the the Bitcoin system for example for the adoption of various soft fork features or even hard fork features […]