Briefing
The core problem addressed is the persistent centralization and censorship risk inherent in current block production models, even those utilizing Proposer-Builder Separation (PBS). This research introduces a formal model for Credibly Neutral Block Production (CNBP), which employs a two-stage commit-reveal cryptographic protocol to enforce neutrality. This mechanism forces block builders to commit to a transaction ordering before the block proposer is known, thereby decoupling the proposer’s ability to censor from the builder’s profit maximization. The single most important implication is the foundational establishment of a truly credibly neutral transaction ordering layer, which is essential for the long-term security and political legitimacy of decentralized finance and public blockchains.
Context
The established Proposer-Builder Separation (PBS) model was introduced to mitigate Maximal Extractable Value (MEV) centralization by separating the roles of block ordering (Builder) and block finalization (Proposer). This design successfully distributed the economic incentives for block construction. However, a critical centralization vector remains → the Proposer retains the final veto power over the Builder’s entire block, allowing for subtle, yet effective, censorship of specific transactions or users by simply rejecting a valid block, a practice known as proposer censorship. This theoretical limitation undermined the goal of a truly credibly neutral system.
Analysis
The paper’s core mechanism is the Two-Phase Committed Block (TPCB) protocol, which acts as a new primitive for block construction. It fundamentally differs from prior PBS by inserting a cryptographic commitment layer. The Builder must first submit a cryptographic commitment (Phase 1 → Commit) to a complete, ordered transaction bundle, which includes a zero-knowledge proof of validity. Only after this commitment is accepted does the Proposer reveal their identity and request the full block (Phase 2 → Reveal).
If the revealed block matches the commitment, it is accepted; otherwise, the Builder is penalized through a slashing mechanism. This structure ensures that the Proposer cannot select a block based on its specific contents, only on its valid, committed state, thereby cryptographically enforcing neutrality and eliminating the proposer’s ability to censor.
Parameters
- Censorship Attack Cost Multiplier → 10x. The mechanism increases the minimum economic cost for a successful, sustained censorship attack by a factor of ten, relative to traditional PBS.
- Commitment Latency Overhead → < 50ms. The additional cryptographic overhead for the two-phase commit is measured at less than 50 milliseconds, ensuring negligible impact on block production time.
Outlook
This formalization provides the necessary cryptographic and game-theoretic primitives to move beyond theoretical PBS to a fully enforced, credibly neutral transaction ordering layer. Future research will focus on integrating the TPCB protocol into existing decentralized sequencer architectures and optimizing the zero-knowledge proof components for even lower latency and greater expressiveness. The real-world application is the unlocking of truly censorship-resistant L2 rollups and the mitigation of systemic political risk for public blockchains, ensuring the protocol’s long-term viability as a neutral public good.
