Fast Deterministically Safe Proof-of-Work Consensus

2025-12-22

AI Summary

Existing permissionless blockchains suffer from fundamental security limitations: Proof-of-Work (PoW) is vulnerable to 51% hash-power attacks, while Proof-of-Stake (PoS) is susceptible to long-range attacks; mainstream solutions either rely on external mechanisms (e.g., social consensus) or provide only probabilistic safety and high latency. Method: We propose Sieve-MMR, the first fully permissionless PoW consensus protocol achieving deterministic safety and constant expected latency. Its core innovation is the Time-Travel-Resilient Broadcast (TTRB) primitive, which adapts the security model of the PoS protocol MMR to the PoW paradigm. Leveraging a black-box deterministic PoW layer and an MMR-inspired state synchronization framework, Sieve-MMR constructs a message layer resilient to both time-travel and long-range attacks. Contribution/Results: Without assuming weak synchrony, external coordination, or social consensus, Sieve-MMR guarantees deterministic finality, constant expected latency, and simultaneous resilience against 51% attacks and long-range attacks.

Technology Category

• Multiagent Systems: Mechanism Design
• Machine Learning: Large Multimodal Models (LMMs)
• Data Mining & Knowledge Management: Representing, Reasoning, and Using Provenance, Trust

Application Category

• Security and Privacy: Blockchains and distributed ledgers
• Economics, Online Markets and Human Computation: Economic aspects of blockchain and cryptocurrencies
• Web Mining and Content Analysis: Content-based information diffusion

Abstract

Permissionless blockchains achieve consensus while allowing unknown nodes to join and leave the system at any time. They typically come in two flavors: proof of work (PoW) and proof of stake (PoS), and both are vulnerable to attacks. PoS protocols suffer from long-range attacks, wherein attackers alter execution history at little cost, and PoW protocols are vulnerable to attackers with enough computational power to subvert execution history. PoS protocols respond by relying on external mechanisms like social consensus; PoW protocols either fall back to probabilistic guarantees, or are slow. We present Sieve-MMR, the first fully-permissionless protocol with deterministic security and constant expected latency that does not rely on external mechanisms. We obtain Sieve-MMR by porting a PoS protocol (MMR) to the PoW setting. From MMR we inherit constant expected latency and deterministic security, and proof-of-work gives us resilience against long-range attacks. The main challenge to porting MMR to the PoW setting is what we call time-travel attacks, where attackers use PoWs generated in the distant past to increase their perceived PoW power in the present. We respond by proposing Sieve, a novel algorithm that implements a new broadcast primitive we dub time-travel-resilient broadcast (TTRB). Sieve relies on a black-box, deterministic PoW primitive to implement TTRB, which we use as the messaging layer for MMR.

Problem

• Develops a permissionless blockchain with deterministic security
• Addresses vulnerabilities in PoW and PoS to long-range attacks
• Introduces time-travel-resilient broadcast to prevent past PoW misuse

Innovation

• Sieve-MMR combines PoW with MMR for deterministic security
• Introduces time-travel-resilient broadcast to counter PoW attacks
• Uses black-box deterministic PoW for constant latency consensus