PQC output type discussion

The recent implementation of BIP-360 P2MR in Bitcoin Core on the regtest network offers new insights into the efficiency and effectiveness of different transaction types within the Bitcoin protocol. The primary focus of this implementation was to analyze spends by comparing identical transactions that varied only in the input side configuration. Specifically, the transactions compared included one input and one P2TR output, utilizing SIGHASH_DEFAULT and an OP_CHECKSIG leaf.

Detailed measurements highlighted significant differences in transaction efficiency between different script paths. For instance, a P2MR transaction with a 2-leaf tree structure weighs 33 bytes, with a total block weight of 513 and a virtual size of 129 bytes. When the tree structure is increased to 4 leaves under the same transaction type, the weight increases marginally to 65 bytes, while the block weight adjusts to 545 and the virtual size to 137 bytes. In contrast, a P2TR script path spend with a similar 2-leaf tree structure reflects identical weights and sizes to the 4-leaf P2MR transaction. Importantly, it was observed that the P2MR witness configuration is consistently 32 bytes lighter than its P2TR counterpart for equivalent spends across various depths, except at a specific measure where the difference slightly increases due to boundary conditions in compact size calculations.

This advantage implies that P2MR can accommodate an additional tree level without increasing cost relative to the P2TR script path, showcasing potential for more efficient blockchain space utilization. This could have implications for future enhancements in Bitcoin's transaction efficiency and cost-effectiveness. Further research and measurements, especially in other configurations on the regtest network, could provide deeper insights into these preliminary findings. For detailed results and discussions regarding these implementations, you can refer to the full documentation and data here.