DROPS

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Invited Talk

DOI: 10.4230/LIPIcs.OPODIS.2024.3

BFT Consensus: From Academic Paper to Mainnet (Invited Talk)

Authors: Alberto Sonnino

Published in: LIPIcs, Volume 324, 28th International Conference on Principles of Distributed Systems (OPODIS 2024)

Abstract

This talk shares our journey in bringing Byzantine Fault Tolerant (BFT) consensus from academic papers to operational blockchain networks. It begins in 2019 with our initial effort as researchers and engineers at Facebook to deploy the HotStuff consensus protocol [Yin et al., 2019] at the heart of the Libra blockchain [Facebook, 2022]. We present how this journey led to modifications from the original theoretical design [Rati Gelashvili et al., 2021] and the eventual migration to DAG-based systems [George Danezis et al., 2022; Alexander Spiegelman et al., 2022], now implemented in the Sui blockchain [The Sui team, 2023] and gaining traction across the blockchain space [Arun et al., 2024; Kushal Babel et al., 2024; Shrestha et al., 2024; Malkhi et al., 2023]. We outline the numerous research and engineering challenges we faced at every step of this journey, describe how we addressed some of these challenges [Bano et al., 2020; Giuliari et al., 2024], and point out which ones remain open questions and require further research. This talk aims to offer a different perspective on BFT consensus, focusing on the needs of real-world blockchains and offering insights that may not be visible from research papers alone.

Cite as

Alberto Sonnino. BFT Consensus: From Academic Paper to Mainnet (Invited Talk). In 28th International Conference on Principles of Distributed Systems (OPODIS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 324, p. 3:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{sonnino:LIPIcs.OPODIS.2024.3,
  author =	{Sonnino, Alberto},
  title =	{{BFT Consensus: From Academic Paper to Mainnet}},
  booktitle =	{28th International Conference on Principles of Distributed Systems (OPODIS 2024)},
  pages =	{3:1--3:1},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-360-7},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{324},
  editor =	{Bonomi, Silvia and Galletta, Letterio and Rivi\`{e}re, Etienne and Schiavoni, Valerio},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2024.3},
  URN =		{urn:nbn:de:0030-drops-225396},
  doi =		{10.4230/LIPIcs.OPODIS.2024.3},
  annote =	{Keywords: BFT Consensus, Blockchain, Real-World}
}

Document

DOI: 10.4230/LIPIcs.OPODIS.2022.18

Make Every Word Count: Adaptive Byzantine Agreement with Fewer Words

Authors: Shir Cohen, Idit Keidar, and Alexander Spiegelman

Published in: LIPIcs, Volume 253, 26th International Conference on Principles of Distributed Systems (OPODIS 2022)

Abstract

Byzantine Agreement (BA) is a key component in many distributed systems. While Dolev and Reischuk have proven a long time ago that quadratic communication complexity is necessary for worst-case runs, the question of what can be done in practically common runs with fewer failures remained open. In this paper we present the first Byzantine Broadcast algorithm with O(n(f+1)) communication complexity in a model with resilience of n = 2t+1, where 0 ≤ f ≤ t is the actual number of process failures in a run. And for BA with strong unanimity, we present the first optimal-resilience algorithm that has linear communication complexity in the failure-free case and a quadratic cost otherwise.

Cite as

Shir Cohen, Idit Keidar, and Alexander Spiegelman. Make Every Word Count: Adaptive Byzantine Agreement with Fewer Words. In 26th International Conference on Principles of Distributed Systems (OPODIS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 253, pp. 18:1-18:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{cohen_et_al:LIPIcs.OPODIS.2022.18,
  author =	{Cohen, Shir and Keidar, Idit and Spiegelman, Alexander},
  title =	{{Make Every Word Count: Adaptive Byzantine Agreement with Fewer Words}},
  booktitle =	{26th International Conference on Principles of Distributed Systems (OPODIS 2022)},
  pages =	{18:1--18:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-265-5},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{253},
  editor =	{Hillel, Eshcar and Palmieri, Roberto and Rivi\`{e}re, Etienne},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2022.18},
  URN =		{urn:nbn:de:0030-drops-176385},
  doi =		{10.4230/LIPIcs.OPODIS.2022.18},
  annote =	{Keywords: Byzantine Agreement, Byzantine Broadcast, Adaptive communication}
}

Document

Invited Talk

DOI: 10.4230/OASIcs.FAB.2022.4

Efficient DAG-Based Consensus (Invited Talk)

Authors: Alberto Sonnino

Published in: OASIcs, Volume 101, 5th International Symposium on Foundations and Applications of Blockchain 2022 (FAB 2022)

Abstract

This talk shows how to build high-performant Byzantine fault-tolerant (BFT) quorum-based consensus cores. The talks starts by challenging the common misconception that the overall communication complexity of the protocol is the key factor determining performance. We instead argue that the bottleneck of many state-of-the-art consensus protocols is their sequential use of the machine’s resources (network, storage, CPU), and that data dissemination is the most resource-intensive task. In light of the above considerations, the first insight to build performant BFT-based consensus cores is to separate the task of reliable transaction dissemination from transaction ordering. We show how to design a new DAG-based mempool protocol, called Narwhal, specialising in high-throughput reliable dissemination and storage of causal histories of transactions. Narwhal tolerates an asynchronous network and maintains high performance despite failures. It is designed to easily scale-out using multiple workers at each validator to concurrently use the machine’s resources (network, storage, CPU), and demonstrates that there is no foreseeable limit to the throughput we can achieve. We then present two ways to leverage Narwhal to achieve consensus. We first (i) present Tusk, a zero-message overhead asynchronous consensus protocol designed to work with Narwhal. Tusk achieves an unprecedented 160,000 tx/s with about 3 seconds latency in a geo-replicated environment. We then (ii) show how any partially-synchronous consensus, such as HotStuff (PODC 19), can be composed with Narwhal to drastically improve its performance. HotStuff running over Narwhal sees its throughput increase from about 2,000 tx/s to over 130,000 tx/s without noticeable latency increase. The talk concludes by illustrating how to properly evaluate performance of BFT-based consensus cores. It highlights the most common mistakes seen in the literature, such as benchmarks with empty transactions (empty load), performance approximation based on LAN-only benchmarks, and using a single burst of input transactions. We then show how to analyse benchmark results using latency-throughput graphs (L-graphs) and SLA-based throughput graphs. Author Bio. I am a system researcher at Mysten Labs, based in London (UK). I previously was a research scientist at Facebook (now called Meta) in the blockchain and cryptography team. Before joining Facebook, I co-founded chainspace.io which built a scalable smart contract platform; the team was then acquired by Facebook. My research interests are in systems security and privacy engineering. My main areas of research include distributed systems, blockchains, and privacy enhancing technologies. I have a special interest in cryptography, and I spend most of my time designing, implementing and evaluating high-performance distributed systems.

Cite as

Alberto Sonnino. Efficient DAG-Based Consensus (Invited Talk). In 5th International Symposium on Foundations and Applications of Blockchain 2022 (FAB 2022). Open Access Series in Informatics (OASIcs), Volume 101, p. 4:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{sonnino:OASIcs.FAB.2022.4,
  author =	{Sonnino, Alberto},
  title =	{{Efficient DAG-Based Consensus}},
  booktitle =	{5th International Symposium on Foundations and Applications of Blockchain 2022 (FAB 2022)},
  pages =	{4:1--4:1},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-248-8},
  ISSN =	{2190-6807},
  year =	{2022},
  volume =	{101},
  editor =	{Tucci-Piergiovanni, Sara and Crooks, Natacha},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.FAB.2022.4},
  URN =		{urn:nbn:de:0030-drops-162712},
  doi =		{10.4230/OASIcs.FAB.2022.4},
  annote =	{Keywords: Consensus protocol, Byzantine Fault Tolerant}
}

Document

DOI: 10.4230/OASIcs.Tokenomics.2021.3

Tuning PoW with Hybrid Expenditure

Authors: Itay Tsabary, Alexander Spiegelman, and Ittay Eyal

Published in: OASIcs, Volume 97, 3rd International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2021)

Abstract

Proof of Work (PoW) is a Sybil-deterrence security mechanism. It introduces an external cost to system participation by requiring computational effort to perform actions. However, since its inception, a central challenge was to tune this cost. Initial designs for deterring spam email and DoS attacks applied overhead equally to honest participants and attackers. Requiring too little effort does not deter attacks, whereas too much encumbers honest participation. This might be the reason it was never widely adopted. Nakamoto overcame this trade-off in Bitcoin by distinguishing desired from malicious behavior and introducing internal rewards for the former. This mechanism gained popularity in securing permissionless cryptocurrencies, using virtual internally-minted tokens for rewards. However, in existing blockchain protocols the internal rewards directly compensate users for (almost) the same value of external expenses. Thus, as the token value soars, so does the PoW expenditure. Bitcoin PoW, for example, already expends as much electricity as Colombia or Switzerland. This amount of resource-guzzling is unsustainable, and hinders even wider adoption of these systems. As such, a prominent alternative named Proof of Stake (PoS) replaces the expenditure requirement with token possession. However, PoS is shun by many cryptocurrency projects, as it is only secure under qualitatively-different assumptions, and the resultant systems are not permissionless. In this work we present Hybrid Expenditure Blockchain (HEB), a novel PoW mechanism. HEB is a generalization of Nakamoto’s protocol that enables tuning the external expenditure by introducing a complementary internal-expenditure mechanism. Thus, for the first time, HEB decouples external expenditure from the reward value. We show a practical parameter choice by which HEB requires significantly less external consumption compare to Nakamoto’s protocol, its resilience against rational attackers is similar, and it retains the decentralized and permissionless nature of the system. Taking the Bitcoin ecosystem as an example, HEB cuts the electricity consumption by half.

Cite as

Itay Tsabary, Alexander Spiegelman, and Ittay Eyal. Tuning PoW with Hybrid Expenditure. In 3rd International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2021). Open Access Series in Informatics (OASIcs), Volume 97, pp. 3:1-3:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{tsabary_et_al:OASIcs.Tokenomics.2021.3,
  author =	{Tsabary, Itay and Spiegelman, Alexander and Eyal, Ittay},
  title =	{{Tuning PoW with Hybrid Expenditure}},
  booktitle =	{3rd International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2021)},
  pages =	{3:1--3:17},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-220-4},
  ISSN =	{2190-6807},
  year =	{2022},
  volume =	{97},
  editor =	{Gramoli, Vincent and Halaburda, Hanna and Pass, Rafael},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.Tokenomics.2021.3},
  URN =		{urn:nbn:de:0030-drops-159008},
  doi =		{10.4230/OASIcs.Tokenomics.2021.3},
  annote =	{Keywords: Blockchain, Proof of work, Cryptocurrency, Environmental impact}
}

Document

DOI: 10.4230/LIPIcs.OPODIS.2021.30

Using Nesting to Push the Limits of Transactional Data Structure Libraries

Authors: Gal Assa, Hagar Meir, Guy Golan-Gueta, Idit Keidar, and Alexander Spiegelman

Published in: LIPIcs, Volume 217, 25th International Conference on Principles of Distributed Systems (OPODIS 2021)

Abstract

Transactional data structure libraries (TDSL) combine the ease-of-programming of transactions with the high performance and scalability of custom-tailored concurrent data structures. They can be very efficient thanks to their ability to exploit data structure semantics in order to reduce overhead, aborts, and wasted work compared to general-purpose software transactional memory. However, TDSLs were not previously used for complex use-cases involving long transactions and a variety of data structures. In this paper, we boost the performance and usability of a TDSL, towards allowing it to support complex applications. A key idea is nesting. Nested transactions create checkpoints within a longer transaction, so as to limit the scope of abort, without changing the semantics of the original transaction. We build a Java TDSL with built-in support for nested transactions over a number of data structures. We conduct a case study of a complex network intrusion detection system that invests a significant amount of work to process each packet. Our study shows that our library outperforms publicly available STMs twofold without nesting, and by up to 16x when nesting is used.

Cite as

Gal Assa, Hagar Meir, Guy Golan-Gueta, Idit Keidar, and Alexander Spiegelman. Using Nesting to Push the Limits of Transactional Data Structure Libraries. In 25th International Conference on Principles of Distributed Systems (OPODIS 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 217, pp. 30:1-30:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{assa_et_al:LIPIcs.OPODIS.2021.30,
  author =	{Assa, Gal and Meir, Hagar and Golan-Gueta, Guy and Keidar, Idit and Spiegelman, Alexander},
  title =	{{Using Nesting to Push the Limits of Transactional Data Structure Libraries}},
  booktitle =	{25th International Conference on Principles of Distributed Systems (OPODIS 2021)},
  pages =	{30:1--30:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-219-8},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{217},
  editor =	{Bramas, Quentin and Gramoli, Vincent and Milani, Alessia},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2021.30},
  URN =		{urn:nbn:de:0030-drops-158058},
  doi =		{10.4230/LIPIcs.OPODIS.2021.30},
  annote =	{Keywords: Transactional Libraries, Nesting}
}

Document

DOI: 10.4230/LIPIcs.DISC.2021.38

In Search for an Optimal Authenticated Byzantine Agreement

Authors: Alexander Spiegelman

Published in: LIPIcs, Volume 209, 35th International Symposium on Distributed Computing (DISC 2021)

Abstract

In this paper, we challenge the conventional approach of state machine replication systems to design deterministic agreement protocols in the eventually synchronous communication model. We first prove that no such protocol can guarantee bounded communication cost before the global stabilization time and propose a different approach that hopes for the best (synchrony) but prepares for the worst (asynchrony). Accordingly, we design an optimistic byzantine agreement protocol that first tries an efficient deterministic algorithm that relies on synchrony for termination only, and then, only if an agreement was not reached due to asynchrony, the protocol uses a randomized asynchronous protocol for fallback that guarantees termination with probability 1. We formally prove that our protocol achieves optimal communication complexity under all network conditions and failure scenarios. We first prove a lower bound of Ω(ft+ t) for synchronous deterministic byzantine agreement protocols, where t is the failure threshold, and f is the actual number of failures. Then, we present a tight upper bound and use it for the synchronous part of the optimistic protocol. Finally, for the asynchronous fallback, we use a variant of the (optimal) VABA protocol, which we reconstruct to safely combine it with the synchronous part. We believe that our adaptive to failures synchronous byzantine agreement protocol has an independent interest since it is the first protocol we are aware of which communication complexity optimally depends on the actual number of failures.

Cite as

Alexander Spiegelman. In Search for an Optimal Authenticated Byzantine Agreement. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 38:1-38:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{spiegelman:LIPIcs.DISC.2021.38,
  author =	{Spiegelman, Alexander},
  title =	{{In Search for an Optimal Authenticated Byzantine Agreement}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{38:1--38:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.38},
  URN =		{urn:nbn:de:0030-drops-148401},
  doi =		{10.4230/LIPIcs.DISC.2021.38},
  annote =	{Keywords: Byzantine agreement, Optimistic, Asynchronous fallback}
}

Document

Brief Announcement

DOI: 10.4230/LIPIcs.DISC.2021.45

Brief Announcement: Using Nesting to Push the Limits of Transactional Data Structure Libraries

Authors: Gal Assa, Hagar Meir, Guy Golan-Gueta, Idit Keidar, and Alexander Spiegelman

Published in: LIPIcs, Volume 209, 35th International Symposium on Distributed Computing (DISC 2021)

Abstract

Transactional data structure libraries (TDSL) combine the ease-of-programming of transactions with the high performance and scalability of custom-tailored concurrent data structures. They can be very efficient thanks to their ability to exploit data structure semantics in order to reduce overhead, aborts, and wasted work compared to general-purpose software transactional memory. However, TDSLs were not previously used for complex use-cases involving long transactions and a variety of data structures. In this paper, we boost the performance and usability of a TDSL, towards allowing it to support complex applications. A key idea is nesting. Nested transactions create checkpoints within a longer transaction, so as to limit the scope of abort, without changing the semantics of the original transaction. We build a Java TDSL with built-in support for nested transactions over a number of data structures. We conduct a case study of a complex network intrusion detection system that invests a significant amount of work to process each packet. Our study shows that our library outperforms publicly available STMs twofold without nesting, and by up to 16x when nesting is used.

Cite as

Gal Assa, Hagar Meir, Guy Golan-Gueta, Idit Keidar, and Alexander Spiegelman. Brief Announcement: Using Nesting to Push the Limits of Transactional Data Structure Libraries. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 45:1-45:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{assa_et_al:LIPIcs.DISC.2021.45,
  author =	{Assa, Gal and Meir, Hagar and Golan-Gueta, Guy and Keidar, Idit and Spiegelman, Alexander},
  title =	{{Brief Announcement: Using Nesting to Push the Limits of Transactional Data Structure Libraries}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{45:1--45:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.45},
  URN =		{urn:nbn:de:0030-drops-148479},
  doi =		{10.4230/LIPIcs.DISC.2021.45},
  annote =	{Keywords: Transactional Libraries}
}

Document

Brief Announcement

DOI: 10.4230/LIPIcs.DISC.2021.57

Brief Announcement: Probabilistic Indistinguishability and The Quality of Validity in Byzantine Agreement

Authors: Guy Goren, Yoram Moses, and Alexander Spiegelman

Published in: LIPIcs, Volume 209, 35th International Symposium on Distributed Computing (DISC 2021)

Abstract

Lower bounds and impossibility results in distributed computing are both intellectually challenging and practically important. Hundreds if not thousands of proofs appear in the literature, but surprisingly, the vast majority of them apply to deterministic algorithms only. Probabilistic protocols have been around for at least four decades and are receiving a lot of attention with the emergence of blockchain systems. Nonetheless, we are aware of only a handful of randomized lower bounds. In this work we provide a formal framework for reasoning about randomized distributed algorithms. We generalize the notion of indistinguishability, the most useful tool in deterministic lower bounds, to apply to a probabilistic setting. We apply this framework to prove a result of independent interest. Namely, we completely characterize the quality of decisions that protocols for a randomized multi-valued Consensus problem can guarantee in an asynchronous environment with Byzantine faults. We use the new notion to prove a lower bound on the guaranteed probability that honest parties will not decide on a possibly bogus value proposed by a malicious party. Finally, we show that the bound is tight by providing a protocol that matches it. This brief announcement consists of an introduction to the full paper [Guy Goren et al., 2020] by the same title. The interested reader is advised to consult the full paper for a detailed exposition.

Cite as

Guy Goren, Yoram Moses, and Alexander Spiegelman. Brief Announcement: Probabilistic Indistinguishability and The Quality of Validity in Byzantine Agreement. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 57:1-57:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{goren_et_al:LIPIcs.DISC.2021.57,
  author =	{Goren, Guy and Moses, Yoram and Spiegelman, Alexander},
  title =	{{Brief Announcement: Probabilistic Indistinguishability and The Quality of Validity in Byzantine Agreement}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{57:1--57:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.57},
  URN =		{urn:nbn:de:0030-drops-148596},
  doi =		{10.4230/LIPIcs.DISC.2021.57},
  annote =	{Keywords: Indistinguishability, probabilistic lower bounds, Byzantine agreement}
}

Document

Invited Talk

DOI: 10.4230/LIPIcs.OPODIS.2020.2

Byzantine Agreement and SMR with Sub-Quadratic Message Complexity (Invited Talk)

Authors: Idit Keidar

Published in: LIPIcs, Volume 184, 24th International Conference on Principles of Distributed Systems (OPODIS 2020)

Abstract

Byzantine Agreement (BA) has been studied for four decades by now, but until recently, has been considered at a fairly small scale. In recent years, however, we begin to see practical use-cases of BA in large-scale systems, which motivates a push for reduced communication complexity. Dolev and Reischuk’s well-known lower bound stipulates that any deterministic algorithm requires Ω(n²) communication in the worst-case, and until fairly recently, almost all randomized algorithms have had at least quadratic complexity as well. This talk will present two new algorithms breaking this barrier. The first part of the talk will consider a fully asynchronous setting, focusing on randomized BA whose safety and liveness guarantees hold with high probability. It will present the first asynchronous Byzantine Agreement algorithm with sub-quadratic communication complexity. This algorithm exploits VRF-based committee sampling, which it adapts for the asynchronous model. The second part of the talk will consider the eventually synchronous model, where BA and State Machine Replication (SMR) can be solved with deterministic safety and liveness guarantees. In this context, randomization is used in order to reduce the expected communication complexity. The talk will present an algorithm for round synchronization, which is a building block for BA and SMR and constitutes the main performance bottleneck therein. It will present an algorithm that, for the first time, achieves round synchronization with expected linear message complexity and expected constant latency. Existing protocols can use this round synchronization algorithm to solve Byzantine SMR with the same asymptotic performance. The first part of the talk is based on joint work with Shir Cohen and Alexander Spiegelman, and the second part of the talk is based on joint work with Oded Naor.

Cite as

Idit Keidar. Byzantine Agreement and SMR with Sub-Quadratic Message Complexity (Invited Talk). In 24th International Conference on Principles of Distributed Systems (OPODIS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 184, p. 2:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{keidar:LIPIcs.OPODIS.2020.2,
  author =	{Keidar, Idit},
  title =	{{Byzantine Agreement and SMR with Sub-Quadratic Message Complexity}},
  booktitle =	{24th International Conference on Principles of Distributed Systems (OPODIS 2020)},
  pages =	{2:1--2:1},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-176-4},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{184},
  editor =	{Bramas, Quentin and Oshman, Rotem and Romano, Paolo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2020.2},
  URN =		{urn:nbn:de:0030-drops-134874},
  doi =		{10.4230/LIPIcs.OPODIS.2020.2},
  annote =	{Keywords: Distributed Computing, Byzantine Agreement}
}

Document

DOI: 10.4230/LIPIcs.OPODIS.2020.9

ACE: Abstract Consensus Encapsulation for Liveness Boosting of State Machine Replication

Authors: Alexander Spiegelman, Arik Rinberg, and Dahlia Malkhi

Published in: LIPIcs, Volume 184, 24th International Conference on Principles of Distributed Systems (OPODIS 2020)

Abstract

With the emergence of attack-prone cross-organization systems, providing asynchronous state machine replication (SMR) solutions is no longer a theoretical concern. This paper presents ACE, a framework for the design of such fault tolerant systems. Leveraging a known paradigm for randomized consensus solutions, ACE wraps existing practical solutions and real-life systems, boosting their liveness under adversarial conditions and, at the same time, promoting load balancing and fairness. Boosting is achieved without modifying the overall design or the engineering of these solutions. ACE is aimed at boosting the prevailing approach for practical fault tolerance. This approach, often named partial synchrony, is based on a leader-based paradigm: a good leader makes progress and a bad leader does no harm. The partial synchrony approach focuses on safety and forgoes liveness under targeted and dynamic attacks. Specifically, an attacker might block specific leaders, e.g., through a denial of service, to prevent progress. ACE provides boosting by running waves of parallel leaders and selecting a winning leader only retroactively, achieving boosting at a linear communication cost increase. ACE is agnostic to the fault model, inheriting it s failure model from the wrapped solution assumptions. As our evaluation shows, an asynchronous Byzantine fault tolerance (BFT) replication system built with ACE around an existing partially synchronous BFT protocol demonstrates reasonable slow-down compared with the base BFT protocol during faultless synchronous scenarios, yet exhibits significant speedup while the system is under attack.

Cite as

Alexander Spiegelman, Arik Rinberg, and Dahlia Malkhi. ACE: Abstract Consensus Encapsulation for Liveness Boosting of State Machine Replication. In 24th International Conference on Principles of Distributed Systems (OPODIS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 184, pp. 9:1-9:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{spiegelman_et_al:LIPIcs.OPODIS.2020.9,
  author =	{Spiegelman, Alexander and Rinberg, Arik and Malkhi, Dahlia},
  title =	{{ACE: Abstract Consensus Encapsulation for Liveness Boosting of State Machine Replication}},
  booktitle =	{24th International Conference on Principles of Distributed Systems (OPODIS 2020)},
  pages =	{9:1--9:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-176-4},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{184},
  editor =	{Bramas, Quentin and Oshman, Rotem and Romano, Paolo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2020.9},
  URN =		{urn:nbn:de:0030-drops-134948},
  doi =		{10.4230/LIPIcs.OPODIS.2020.9},
  annote =	{Keywords: Framework, Asynchronous, Consensus boosting, State Machine Replication}
}

Document

DOI: 10.4230/LIPIcs.DISC.2020.25

Not a COINcidence: Sub-Quadratic Asynchronous Byzantine Agreement WHP

Authors: Shir Cohen, Idit Keidar, and Alexander Spiegelman

Published in: LIPIcs, Volume 179, 34th International Symposium on Distributed Computing (DISC 2020)

Abstract

King and Saia were the first to break the quadratic word complexity bound for Byzantine Agreement in synchronous systems against an adaptive adversary, and Algorand broke this bound with near-optimal resilience (first in the synchronous model and then with eventual-synchrony). Yet the question of asynchronous sub-quadratic Byzantine Agreement remained open. To the best of our knowledge, we are the first to answer this question in the affirmative. A key component of our solution is a shared coin algorithm based on a VRF. A second essential ingredient is VRF-based committee sampling, which we formalize and utilize in the asynchronous model for the first time. Our algorithms work against a delayed-adaptive adversary, which cannot perform after-the-fact removals but has full control of Byzantine processes and full information about communication in earlier rounds. Using committee sampling and our shared coin, we solve Byzantine Agreement with high probability, with a word complexity of Õ(n) and O(1) expected time, breaking the O(n²) bit barrier for asynchronous Byzantine Agreement.

Cite as

Shir Cohen, Idit Keidar, and Alexander Spiegelman. Not a COINcidence: Sub-Quadratic Asynchronous Byzantine Agreement WHP. In 34th International Symposium on Distributed Computing (DISC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 179, pp. 25:1-25:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{cohen_et_al:LIPIcs.DISC.2020.25,
  author =	{Cohen, Shir and Keidar, Idit and Spiegelman, Alexander},
  title =	{{Not a COINcidence: Sub-Quadratic Asynchronous Byzantine Agreement WHP}},
  booktitle =	{34th International Symposium on Distributed Computing (DISC 2020)},
  pages =	{25:1--25:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-168-9},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{179},
  editor =	{Attiya, Hagit},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2020.25},
  URN =		{urn:nbn:de:0030-drops-131034},
  doi =		{10.4230/LIPIcs.DISC.2020.25},
  annote =	{Keywords: shared coin, Byzantine Agreement, VRF, sub-quadratic consensus protocol}
}

Document

DOI: 10.4230/LIPIcs.OPODIS.2019.4

FairLedger: A Fair Blockchain Protocol for Financial Institutions

Authors: Kfir Lev-Ari, Alexander Spiegelman, Idit Keidar, and Dahlia Malkhi

Published in: LIPIcs, Volume 153, 23rd International Conference on Principles of Distributed Systems (OPODIS 2019)

Abstract

Financial institutions nowadays are looking into technologies for permissioned blockchains. A major effort in this direction is Hyperledger, an open source project hosted by the Linux Foundation and backed by a consortium of over a hundred companies. A key component in permissioned blockchain protocols is a byzantine fault tolerant (BFT) consensus engine that orders transactions. However, currently available BFT solutions in Hyperledger (as well as in the literature at large) are inadequate for financial settings; they are not designed to ensure fairness or to tolerate the selfish behavior that inevitably arises when financial institutions strive to maximize their own profit. We present FairLedger, a permissioned BFT blockchain protocol, which is fair, deigned to deal with rational behavior, and, no less important, easy to understand and implement. Our secret sauce is a new communication abstraction called detectable all-to-all (DA2A), which allows us to detect players (byzantine or rational) that deviate from the protocol and punish them. We implement FairLedger in the Hyperledger open source project using the Iroha framework - one of the biggest projects therein. To evaluate FairLegder’s performance, we also implement it in the PBFT framework and compare the two protocols. Our results show that in failure-free scenarios in wide-area settings, FairLedger achieves better throughput than both Iroha’s implementation and PBFT.

Cite as

Kfir Lev-Ari, Alexander Spiegelman, Idit Keidar, and Dahlia Malkhi. FairLedger: A Fair Blockchain Protocol for Financial Institutions. In 23rd International Conference on Principles of Distributed Systems (OPODIS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 153, pp. 4:1-4:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{levari_et_al:LIPIcs.OPODIS.2019.4,
  author =	{Lev-Ari, Kfir and Spiegelman, Alexander and Keidar, Idit and Malkhi, Dahlia},
  title =	{{FairLedger: A Fair Blockchain Protocol for Financial Institutions}},
  booktitle =	{23rd International Conference on Principles of Distributed Systems (OPODIS 2019)},
  pages =	{4:1--4:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-133-7},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{153},
  editor =	{Felber, Pascal and Friedman, Roy and Gilbert, Seth and Miller, Avery},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2019.4},
  URN =		{urn:nbn:de:0030-drops-117904},
  doi =		{10.4230/LIPIcs.OPODIS.2019.4},
  annote =	{Keywords: Blockchain, Fairness, Byzantine fault tolerance, Rational players, Equilibrium}
}

Document

DOI: 10.4230/LIPIcs.DISC.2018.11

Integrated Bounds for Disintegrated Storage

Authors: Alon Berger, Idit Keidar, and Alexander Spiegelman

Published in: LIPIcs, Volume 121, 32nd International Symposium on Distributed Computing (DISC 2018)

Abstract

We point out a somewhat surprising similarity between non-authenticated Byzantine storage, coded storage, and certain emulations of shared registers from smaller ones. A common characteristic in all of these is the inability of reads to safely return a value obtained in a single atomic access to shared storage. We collectively refer to such systems as disintegrated storage, and show integrated space lower bounds for asynchronous regular wait-free emulations in all of them. In a nutshell, if readers are invisible, then the storage cost of such systems is inherently exponential in the size of written values; otherwise, it is at least linear in the number of readers. Our bounds are asymptotically tight to known algorithms, and thus justify their high costs.

Cite as

Alon Berger, Idit Keidar, and Alexander Spiegelman. Integrated Bounds for Disintegrated Storage. In 32nd International Symposium on Distributed Computing (DISC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 121, pp. 11:1-11:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{berger_et_al:LIPIcs.DISC.2018.11,
  author =	{Berger, Alon and Keidar, Idit and Spiegelman, Alexander},
  title =	{{Integrated Bounds for Disintegrated Storage}},
  booktitle =	{32nd International Symposium on Distributed Computing (DISC 2018)},
  pages =	{11:1--11:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-092-7},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{121},
  editor =	{Schmid, Ulrich and Widder, Josef},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2018.11},
  URN =		{urn:nbn:de:0030-drops-98009},
  doi =		{10.4230/LIPIcs.DISC.2018.11},
  annote =	{Keywords: storage, coding, lower bounds, space complexity, register emulations}
}

Document

DOI: 10.4230/LIPIcs.OPODIS.2017.25

Solida: A Blockchain Protocol Based on Reconfigurable Byzantine Consensus

Authors: Ittai Abraham, Dahlia Malkhi, Kartik Nayak, Ling Ren, and Alexander Spiegelman

Published in: LIPIcs, Volume 95, 21st International Conference on Principles of Distributed Systems (OPODIS 2017)

Abstract

The decentralized cryptocurrency Bitcoin has experienced great success but also encountered many challenges. One of the challenges has been the long confirmation time. Another chal- lenge is the lack of incentives at certain steps of the protocol, raising concerns for transaction withholding, selfish mining, etc. To address these challenges, we propose Solida, a decentralized blockchain protocol based on reconfigurable Byzantine consensus augmented by proof-of-work. Solida improves on Bitcoin in confirmation time, and provides safety and liveness assuming the adversary control less than (roughly) one-third of the total mining power.

Cite as

Ittai Abraham, Dahlia Malkhi, Kartik Nayak, Ling Ren, and Alexander Spiegelman. Solida: A Blockchain Protocol Based on Reconfigurable Byzantine Consensus. In 21st International Conference on Principles of Distributed Systems (OPODIS 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 95, pp. 25:1-25:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{abraham_et_al:LIPIcs.OPODIS.2017.25,
  author =	{Abraham, Ittai and Malkhi, Dahlia and Nayak, Kartik and Ren, Ling and Spiegelman, Alexander},
  title =	{{Solida: A Blockchain Protocol Based on Reconfigurable Byzantine Consensus}},
  booktitle =	{21st International Conference on Principles of Distributed Systems (OPODIS 2017)},
  pages =	{25:1--25:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-061-3},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{95},
  editor =	{Aspnes, James and Bessani, Alysson and Felber, Pascal and Leit\~{a}o, Jo\~{a}o},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2017.25},
  URN =		{urn:nbn:de:0030-drops-86409},
  doi =		{10.4230/LIPIcs.OPODIS.2017.25},
  annote =	{Keywords: Cryptocurrency, Blockchain, Byzantine fault tolerance, Reconfiguration}
}

Document

DOI: 10.4230/LIPIcs.DISC.2017.40

Dynamic Reconfiguration: Abstraction and Optimal Asynchronous Solution

Authors: Alexander Spiegelman, Idit Keidar, and Dahlia Malkhi

Published in: LIPIcs, Volume 91, 31st International Symposium on Distributed Computing (DISC 2017)

Abstract

Providing clean and efficient foundations and tools for reconfiguration is a crucial enabler for distributed system management today. This work takes a step towards developing such foundations. It considers classic fault-tolerant atomic objects emulated on top of a static set of fault-prone servers, and turns them into dynamic ones. The specification of a dynamic object extends the corresponding static (non-dynamic) one with an API for changing the underlying set of fault-prone servers. Thus, in a dynamic model, an object can start in some configuration and continue in a different one. Its liveness is preserved through the reconfigurations it undergoes, tolerating a versatile set of faults as it shifts from one configuration to another. In this paper we present a general abstraction for asynchronous reconfiguration, and exemplify its usefulness for building two dynamic objects: a read/write register and a max-register. We first define a dynamic model with a clean failure condition that allows an administrator to reconfigure the system and switch off a server once the reconfiguration operation removing it completes. We then define the Reconfiguration abstraction and show how it can be used to build dynamic registers and max-registers. Finally, we give an optimal asynchronous algorithm implementing the Reconfiguration abstraction, which in turn leads to the first asynchronous (consensus-free) dynamic register emulation with optimal complexity. More concretely, faced with n requests for configuration changes, the number of configurations that the dynamic register is implemented over is n; and the complexity of each client operation is O(n).

Cite as

Alexander Spiegelman, Idit Keidar, and Dahlia Malkhi. Dynamic Reconfiguration: Abstraction and Optimal Asynchronous Solution. In 31st International Symposium on Distributed Computing (DISC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 91, pp. 40:1-40:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{spiegelman_et_al:LIPIcs.DISC.2017.40,
  author =	{Spiegelman, Alexander and Keidar, Idit and Malkhi, Dahlia},
  title =	{{Dynamic Reconfiguration: Abstraction and Optimal Asynchronous Solution}},
  booktitle =	{31st International Symposium on Distributed Computing (DISC 2017)},
  pages =	{40:1--40:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-053-8},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{91},
  editor =	{Richa, Andr\'{e}a},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2017.40},
  URN =		{urn:nbn:de:0030-drops-79673},
  doi =		{10.4230/LIPIcs.DISC.2017.40},
  annote =	{Keywords: Reconfiguration, Dynamic Objects, Optimal Algorithm}
}

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