Taxonomy of Transaction Fees in Smart Contracts

In the realm of cryptocurrency transactions, particularly within Bitcoin, there's an ongoing discussion regarding the nature and implications of transaction fees. Traditionally, transaction fees are implicit, calculated as the difference between the sum of inputs and outputs. However, a concept worth exploring is that of explicit fee outputs, where fees paid to miners are delineated as separate outputs in a transaction. This method diverges from the conventional implicit fee structure by clearly specifying the fee amount, although it demands additional block space and thus, must adequately compensate for the miner's lost opportunity cost.

One critical aspect often overlooked in discussions about explicit fee outputs is their potential bypass of coinbase maturity restrictions. Coinbase maturity plays a pivotal role in ensuring Bitcoin's security by imposing a waiting period before newly mined coins can be spent, thereby preventing various forms of abuse and enhancing network stability. Explicit fees that do not adhere to these maturity constraints could introduce risks to the network's integrity.

There's speculation on the future necessity of adopting explicit "anyone_can_spend" fee outputs, especially in scenarios where the Bitcoin network faces higher instances of re-organization or operates without subsidy. Such outputs, potentially scripted to emulate coinbase maturity rules (e.g., with a simple output script like 100 op_csv), might offer a solution for securing transactions under these conditions. Users, particularly those receiving funds, could leverage Child Pays for Parent (CPFP) strategies to attach these outputs to their incoming transactions, ensuring security and adherence to desired transaction protocols in a post-subsidy landscape.