Combined CTV/APO into minimal TXHASH+CSFS

The discussion focuses on the proposed OP_TEMPLATEHASH, a novel concept aimed at enhancing Bitcoin's scripting capabilities, particularly in conjunction with OP_CHECKSIGFROMSTACK(VERIFY) to achieve APO behavior, excluding APO|SINGLE. This mechanism involves creating hashes that commit to various transaction elements including the transaction version, the number of outputs, and the outputs themselves. The hashing process is intricately designed to accommodate different groups of input data such as prevout details, prevscript attributes, sequence numbers, and specific data from the input being validated like the script, annex, and other relevant information. Additionally, the proposal outlines conditions under which locktime and scriptsigs are considered.

A notable aspect of this proposal is the introduction of paired hashing modes, delineated by a numeric hash_mode. These modes are designed to selectively include or exclude certain data in the hash calculation process. The modes vary in their inclusion criteria; for instance, even modes do not hash annex, whereas odd modes do. The detailed breakdown of these modes reveals a systematic approach to hashing, ranging from basic inclusion of locktime and sequence numbers to more complex combinations that incorporate prevscript, script, and scriptsigs data. Each mode is crafted to address specific requirements and scenarios, such as the need to avoid quadratic hashing issues by constraining all other inputs' prevouts, a strategy particularly applied to one input within a transaction.

This comprehensive hashing framework aims to bolster transaction security and flexibility within the Bitcoin network. By meticulously defining the input data groups and hashing modes, the proposal lays the groundwork for more nuanced and secure transaction verification processes. The emphasis on selective data inclusion within the hashes underscores a thoughtful approach to optimizing transaction validation without compromising on security.