The Four Horsemen

The unified specification titled "The Four Horsemen (of the Blockchain)" proposes an innovative approach to Bitcoin coinjoin protocols by integrating various existing technologies into a single, flexible, and decentralized system. This spec aims to enhance on-chain privacy for Bitcoin transactions while minimizing additional fees and optimizing block space usage. It addresses the fragmentation and liquidity issues prevalent in current systems by providing a client that adapts to user needs and accesses a shared liquidity pool.

The specification includes several subprotocols: WabiSabi, ZeroLink, JoinMarket, and PayJoin, each contributing unique strengths to the overall system. WabiSabi facilitates UTXO consolidation without revealing ownership, supports privacy-maximized spending, and is integrated into Wasabi Wallet v2.0+. ZeroLink focuses on efficient remixing with DoS protection and was previously implemented in Wasabi Wallet v1.1+. JoinMarket offers on-demand liquidity, particularly beneficial for large-volume traders, and incorporates mechanisms like Proof of Discrete Log Equivalence to enhance privacy. PayJoin enhances third-party privacy in direct transactions and is supported by multiple wallet versions, including the ones found at Payjoin v1 and Payjoin v2.

Each subprotocol addresses specific weaknesses and costs associated with Bitcoin transactions. For example, WabiSabi, although scalable and private, suffers from slow transaction times and higher per-transaction costs. ZeroLink restricts amount flexibility, potentially leaving unspendable change. JoinMarket, while offering passive liquidity benefits, involves trust in the taker. PayJoin provides improved privacy but introduces points of failure through its reliance on both transaction parties.

Incentive structures and user experience (UX) integration are also thoroughly analyzed, showing how these subprotocols interlink to mitigate their individual limitations and provide comprehensive privacy solutions. Users are motivated to use combinations of these technologies depending on their specific needs—whether seeking cost efficiency, liquidity, or privacy maximization.

This protocol design not only anticipates future enhancements but also requires wallet synchronization via compact block filters or a full node by default, ensuring a robust and user-focused implementation. The document remains open for community feedback, emphasizing the ongoing development and refinement process aimed at achieving optimal privacy and utility in Bitcoin transactions.