AT2: Asynchronous Trustworthy Transfers (1812.10844v3)

Abstract: Many blockchain-based protocols, such as Bitcoin, implement a decentralized asset transfer (or exchange) system. As clearly stated in the original paper by Nakamoto, the crux of this problem lies in prohibiting any participant from engaging in double-spending. There seems to be a common belief that consensus is necessary for solving the double-spending problem. Indeed, whether it is for a permissionless or a permissioned environment, the typical solution uses consensus to build a totally ordered ledger of submitted transfers. In this paper we show that this common belief is false: consensus is not needed to implement of a decentralized asset transfer system. We do so by introducing AT2 (Asynchronous Trustworthy Transfers), a class of consensusless algorithms. To show formally that consensus is unnecessary for asset transfers, we consider this problem first in the shared-memory context. We introduce AT2${SM}$, a wait-free algorithm that asynchronously implements asset transfer in the read-write shared-memory model. In other words, we show that the consensus number of an asset-transfer object is one. In the message passing model with Byzantine faults, we introduce a generic asynchronous algorithm called AT2${MP}$ and discuss two instantiations of this solution. First, AT2${D}$ ensures deterministic guarantees and consequently targets a small scale deployment (tens to hundreds of nodes), typically for a permissioned environment. Second, AT2${P}$ provides probabilistic guarantees and scales well to a very large system size (tens of thousands of nodes), ensuring logarithmic latency and communication complexity. Instead of consensus, we construct AT2${D}$ and AT2${P}$ on top of a broadcast primitive with causal ordering guarantees offering deterministic and probabilistic properties, respectively.

• The paper challenges the necessity of consensus for decentralized asset transfers, introducing AT2 as a class of consensusless algorithms to solve the double-spending problem.
• It presents two algorithms: SM for shared-memory environments and MP for message passing with Byzantine faults, demonstrating secure transfers without requiring total ordering or traditional consensus.
• Performance evaluations show AT2 algorithms achieve significantly higher throughput (1.5x-6x) and lower latency (up to 2x) compared to consensus-based systems, indicating practical efficiency gains.

Asynchronous Trustworthy Transfers (AT2): A New Paradigm for Decentralized Asset Transfers

In the landscape of blockchain protocols and decentralized asset transfer systems, the conventional wisdom has been that achieving consensus is a necessary condition to prevent double-spending attacks. This paper, authored by Rachid Guerraoui et al., challenges this entrenched belief by demonstrating that it is possible to implement a decentralized asset transfer system without resorting to consensus. The authors introduce AT2 (Asynchronous Trustworthy Transfers), a class of consensusless algorithms, which fundamentally alters the perceived need for consensus in achieving secure and efficient asset transfers.

Key Contributions and Findings

1. Revisiting Consensus Necessity: The paper begins by questioning the necessity of consensus for asset transfers, which has been a cornerstone of decentralized systems like Bitcoin. Through theoretical analysis, the authors show that a strong enough ordering is not required to solve the double-spending problem, a foundational issue in these systems.
2. SM Algorithm for Shared-Memory Context: To establish their claim, the authors present the SM algorithm, which operates in a read-write shared-memory model. Through this algorithm, they demonstrate that the consensus number of an asset-transfer object is one, indicating that consensus is not required for implementing decentralized transfers, as opposed to traditional models that rely heavily on consensus-driven mechanisms to serialize operations.
3. MP Algorithm for Message Passing with Byzantine Faults: In environments with Byzantine faults, the authors propose the MP algorithm, utilizing secure broadcast with causal ordering rather than total ordering. This innovative approach, bypassing consensus, ensures efficient communication with both deterministic and probabilistic guarantees. Two variants of the MP algorithm are proposed for different deployment scales, ensuring flexibility across system sizes.
4. Performance Metrics and Comparisons: The paper provides extensive performance evaluations, showcasing that AT2 algorithms achieve a throughput improvement ranging from 1.5x to 6x compared to consensus-based counterparts, alongside reducing latency by up to 2x in scenarios with up to 100 replicas. This highlights the practical efficiencies gained by sidestepping consensus.

Implications and Future Directions

The introduction of AT2 offers several implications for both theory and practical implementations of decentralized systems:

• Simplicity and Efficiency: AT2's algorithms are notably simpler than traditional consensus-based solutions, leading to easier implementation and maintenance while achieving superior performance metrics.
• Scalability in Decentralized Systems: By demonstrating the viability of consensusless algorithms, this work paves the way for scaling decentralized systems more efficiently, which is particularly pertinent as these systems are leveraged for increasing transaction volumes and applications beyond cryptocurrencies.
• Theoretical Foundations: The assertion and proof that asset-transfer objects can function without consensus invite a re-evaluation of other digital object models where consensus has been assumed necessary, potentially revising numerous system designs.
• Extension to Permissionless Systems: Although the primary focus is on permissioned configurations, the principles introduced could be adapted for permissionless settings, prompting further research into designing fully decentralized systems with inherent resistance to Sybil attacks and other forms of Byzantine behavior.

Through this insightful exposition, Rachid Guerraoui and his colleagues offer a novel framework for decentralized asset transfer systems, redefining assumptions about consensus and proposing pathways for future developments in blockchain and distributed computing infrastructures. The development of consensusless systems marks a significant theoretical and practical advance, potentially influencing a broad array of distributed applications.