Luca Zanolini

Luca Zanolini

Research Scientist at the Ethereum Foundation. PhD in Computer Science.

Asymmetric Asynchronous Byzantine Consensus

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An important element of every blockchain network is its protocol for reaching consensus. In traditional, permissioned consensus protocols, all involved processes adhere to a global, symmetric failure model, typically only defined by bounds on the number of faulty processes. More flexible trust assumptions have recently been considered, especially in connection with blockchains. With asymmetric trust, in particular, a process is free to choose which other processes it trusts and which ones might collude against it.

Cachin and Tackmann (OPODIS 2019) introduced asymmetric quorum systems as a generalization of Byzantine quorum systems, which are the key abstraction for realizing consensus in a system with symmetric trust. This paper shows how to realize randomized signature-free asynchronous Byzantine consensus with asymmetric quorums. This results in an optimal consensus protocol with subjective, asymmetric trust and constant expected running time, which is suitable for applications in blockchain networks.

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3-Slot-Finality: SSF is not about “Single” Slot

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In this post, we introduce 3-Slot Finality (3SF), a protocol designed to finalize blocks proposed by an honest proposer within 3 slots when network latency is bounded by a known value Δ – even if subsequent proposers might be dishonest – while requiring only one voting phase per slot. This approach contrasts with previously proposed protocols for Single-Slot Finality 14, which require three voting phases per slot to finalize an honestly proposed block within a single slot resulting in longer slot time. We also show that 3SF guarantees all the key properties expected from SSF, offering an efficient and practical alternative that reduces overhead while ensuring fast and predictable block finalization within a few slots, and a shorter practical slot time (as voting phases take practically longer than other phases). As a result, our protocol achieves a shorter expected confirmation time compared to the previously proposed protocol, at the expense of a slight delay in block finalization, which extends to the time required to propose three additional blocks, rather than finalizing before the next block proposal. However, we believe that a shorter expected confirmation time could be sufficient for most users. Also, slot time is a crucial parameter affecting economic leakage in on-chain automated market makers (AMMs) due to arbitrage. Specifically, arbitrage profits – and consequently, liquidity providers’ (LP) losses – are proportional to the square root of slot time. Therefore, reducing slot time is highly desirable for financial applications on smart contract blockchains. Finally, we show that we can make a further trade-off: we can increase the number of voting phases to two, without this affecting the actual slot length, and achieve finalization in only two slots.

Quorum systems in permissionless networks

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Fail-prone systems, and their quorum systems, are useful tools for the design of distributed algorithms. However, fail-prone systems as studied so far require every process to know the full system membership in order to guarantee safety through globally intersecting quorums. Thus, they are of little help in an open, permissionless setting, where such knowledge may not be available. We propose to generalize the theory of fail-prone systems to make it applicable to permissionless systems. We do so by enabling processes not only to make assumptions about failures, but also to make assumptions about the assumptions of other processes. Thus, by transitivity, processes that do not even know of any common process may nevertheless have intersecting quorums and solve, for example, reliable broadcast. Our model generalizes existing models such as the classic fail-prone system model [Malkhi and Reiter, 1998] and the asymmetric fail-prone system model [Cachin and Tackmann, OPODIS 2019]. Moreover, it gives a characterization with standard formalism of the model used by the Stellar blockchain.

Quick Fair Order

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Leader-based protocols for consensus, i.e., atomic broadcast, allow some processes to unilaterally affect the final order of transactions. This has become a problem for blockchain networks and decentralized finance because it facilitates front-running and other attacks. To address this, order fairness for payload messages has be en introduced recently as a new safety property for atomic broadcast complementing traditional agreement and liveness. We relate order fairness to the standard validity notions for consensus protocols and highlight some limitations with the existing formalization. Based on this, we introduce a new differential order fairness property that fixes these issues. We also present the quick order-fair atomic broadcast protocol that guarantees payload message delivery in a differentially fair order and is much more efficient than existing order-fair consensus protocols. It works for asynchronous and for eventually synchronous networks with optimal resilience, tolerating corruptions of up to one third of the processes. Previous solutions required there to be less than one fourth of faults. Furthermore, our protocol incurs only quadratic cost, in terms of amortized message complexity per delivered payload.