Post-Signature Cross-Input Scripting Using the Taproot Annex

The proposed idea revolves around introducing a cross-input scripting capability into Bitcoin, enabling users to commit to additional spending criteria at the time of signing a transaction. This concept seeks to enrich the functionality of Bitcoin transactions by allowing commitments like timelocks, signatures from other users, or CTV templates post-signature. The mechanism for this feature involves the use of the taproot annex, which would facilitate the inclusion of optional script commitments that can be evaluated after the signature has been made. These script commitments, known as subscripts, could encompass various functionalities such as delegated signatures, post-signature timelocks, and transaction templating for potential future covenants.

The structure of these subscripts is detailed, highlighting the protocol tag and TLV (Type-Length-Value) encoding to differentiate between public subscripts, redeem subscripts, and uninterpreted data. The evaluation of these subscripts would only occur if the regular script execution succeeds, ensuring backward compatibility with existing Bitcoin scripts. This design aims to maintain the integrity of the current system while introducing new programmable capabilities.

The implementation of such a feature would necessitate a soft fork in the Bitcoin network, prompting the author to seek feedback on several fronts. Questions are raised regarding the community's perception of the value of this functionality, interest in modifying the consensus to incorporate taproot annex meanings, potential security vulnerabilities, and suggestions for structural improvements. The call for feedback underscores the proposal's early stage and the desire to refine it through community input, emphasizing the importance of collective insight in the evolution of Bitcoin's scripting capabilities.