Silent Payments coinbase

The discussion revolves around the technical specifics of implementing BIP34 height encoding in coinbase scriptSig construction, focusing on the practical implications for transaction data space. When encoding the block height according to BIP34 requirements, which involves a 2-100 byte consensus limit, a significant amount of space is utilized just for the height encoding. Presently, with the heights observed in both mainnet and testnet4 exceeding 65,536 but remaining below 16,777,216, a total of four bytes are required. This comprises one byte for push length and three bytes for data. Additionally, there could be scenarios necessitating a fifth padding byte to prevent misinterpretation of the height's high bit as a sign bit.

Given these constraints, the actual available space for extranonce within the coinbase transaction is reduced. Initially, it might seem that after accounting for a typical 34-byte A_send (address sending value), there would be plenty of space left from the maximum allowed 100 bytes. However, considering the space taken by BIP34 encoding (currently 4 bytes, potentially growing to 5 bytes as the block height increases beyond 16,777,215), only about 62 bytes remain available for the extranonce. This clarification underscores that while current space might seem adequate, the increasing cost of BIP34 encoding could impose tighter constraints on extranonce size in the future.