On Tue, Dec 8, 2015 at 5:41 PM, Mark Friedenbach via bitcoin-dev < firstname.lastname@example.org> wrote:
> A far better place than the generation transaction (which I assume means > coinbase transaction?) is the last transaction in the block. That allows > you to save, on average, half of the hashes in the Merkle tree. > This trick can be improved by only using certain tx counts. If the number of transactions is limited to a power of 2 (other than the extra transactions), then you get a path of length zero. The number of non-zero bits in the tx count determings how many digests are required. https://github.com/TierNolan/bips/blob/aux_header/bip-aux-header.mediawiki This gets the benefit of a soft-fork, while also keeping the proof lengths small. The linked bip has a 105 byte overhead for the path. The cost is that only certain transaction counts are allowed. In the worst case, 12.5% of transactions would have to be left in the memory pool. This means around 7% of transactions would be delayed until the next block. Blank transactions (or just transactions with low latency requirements) could be used to increase the count so that it is raised to one of the valid numbers. Managing the UTXO set to ensure that there is at least one output that pays to OP_TRUE is also a hassle.
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