[taproot][script] 4-leaf Taproot Merkle Tree in Python: testnet implementation + control block analysis

His work involves the construction of a Taproot address that incorporates four distinct script leaves: an OP_SHA256 + OP_EQUAL hashlock, a 2-of-2 OP_CHECKMULTISIG, an OP_CSV + OP_CHECKSIG, and an OP_CHECKSIG. This Taproot address supports five different spending paths, which include four script path spends and one key path spend. These spending methods have been thoroughly tested and validated on the Bitcoin testnet, offering a comprehensive look at their implementation and execution.

The core of Zhang's project is a hands-on demonstration of constructing a 4-leaf Taproot Merkle tree in Python, followed by a detailed examination of the Taproot output key tweaking process, which involves combining an internal key with the root of the script tree. Furthermore, he provides an in-depth byte-by-byte analysis of the control blocks, which play a crucial role in the validation of Taproot transactions. This analysis covers the structure of control blocks, including the control byte that dictates the leaf version and parity, the internal public key, and the ordered Merkle sibling hashes.

Zhang's work does not stop at theoretical explanations; it extends into practical demonstrations of how these control blocks are constructed and verified against the Taproot output using real-world testnet spends. Additionally, he offers insights into the witness stack, presenting a clear visualization of script execution and how each script path operates during runtime.

For those interested in exploring the technical depths of this project, Zhang has made available several resources. These include two parts of his in-depth analysis published on Medium (Part 1 and Part 2), and a GitHub repository containing the code, testnet transactions, and detailed diagrams (GitHub repo). This comprehensive collection of resources serves as a pedagogical example for developers looking to deepen their understanding of non-trivial Taproot path spends, script trees, control blocks, and Tapscript execution through hands-on experimentation.

Zhang's contribution to the Bitcoin development community is significant, providing both a foundation and an advanced exploration of Taproot functionalities. He encourages feedback and discussion on various aspects of his project, including control block design, merkle path optimization, spend path visibility versus privacy tradeoffs, and tooling for visualizing script-path execution and verification. This initiative reflects a strong commitment to fostering a deeper understanding and broader application of Bitcoin's scripting capabilities among developers.