9 Search Results for "Losa, Giuliano"

Document
DOI: 10.4230/LIPIcs.DISC.2023.28
Quorum Subsumption for Heterogeneous Quorum Systems

Authors: Xiao Li, Eric Chan, and Mohsen Lesani

Published in: LIPIcs, Volume 281, 37th International Symposium on Distributed Computing (DISC 2023)

Abstract
Byzantine quorum systems provide higher throughput than proof-of-work and incur modest energy consumption. Further, their modern incarnations incorporate personalized and heterogeneous trust. Thus, they are emerging as an appealing candidate for global financial infrastructure. However, since their quorums are not uniform across processes anymore, the properties that they should maintain to support abstractions such as reliable broadcast and consensus are not well-understood. It has been shown that the two properties quorum intersection and availability are necessary. In this paper, we prove that they are not sufficient. We then define the notion of quorum subsumption, and show that the three conditions together are sufficient: we present reliable broadcast and consensus protocols, and prove their correctness for quorum systems that provide the three properties.
Cite as

Xiao Li, Eric Chan, and Mohsen Lesani. Quorum Subsumption for Heterogeneous Quorum Systems. In 37th International Symposium on Distributed Computing (DISC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 281, pp. 28:1-28:19, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2023)

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@InProceedings{li_et_al:LIPIcs.DISC.2023.28,
  author =	{Li, Xiao and Chan, Eric and Lesani, Mohsen},
  title =	{{Quorum Subsumption for Heterogeneous Quorum Systems}},
  booktitle =	{37th International Symposium on Distributed Computing (DISC 2023)},
  pages =	{28:1--28:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-301-0},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{281},
  editor =	{Oshman, Rotem},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2023.28},
  URN =		{urn:nbn:de:0030-drops-191541},
  doi =		{10.4230/LIPIcs.DISC.2023.28},
  annote =	{Keywords: Distributed Systems, Impossibility Results, Byzantine fault tolerance}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2023.41
Brief Announcement: Byzantine Consensus Under Dynamic Participation with a Well-Behaved Majority

Authors: Eli Gafni and Giuliano Losa

Published in: LIPIcs, Volume 281, 37th International Symposium on Distributed Computing (DISC 2023)

Abstract
In a permissionless system like Ethereum, participation may fluctuate dynamically as some participants unpredictably go offline and some others come back online. In such an environment, traditional Byzantine fault-tolerant consensus algorithms may stall - even in the absence of failures - because they rely on the availability of fixed-sized quorums. The sleepy model formally captures the main requirements for solving consensus under dynamic participation, and several algorithms solve consensus with probabilistic safety in this model assuming that, at any time, more than half of the online participants are well behaved. However, whether safety can be ensured deterministically under these assumptions, especially with constant latency, remained an open question. Assuming a constant adversary, we answer in the positive by presenting a consensus algorithm that achieves deterministic safety and constant latency in expectation. In the full version of this paper, we also present a second algorithm which obtains both deterministic safety and liveness, but is likely only of theoretical interest because of its high round and message complexity. Both algorithms are striking in their simplicity.
Cite as

Eli Gafni and Giuliano Losa. Brief Announcement: Byzantine Consensus Under Dynamic Participation with a Well-Behaved Majority. In 37th International Symposium on Distributed Computing (DISC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 281, pp. 41:1-41:7, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2023)

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@InProceedings{gafni_et_al:LIPIcs.DISC.2023.41,
  author =	{Gafni, Eli and Losa, Giuliano},
  title =	{{Brief Announcement: Byzantine Consensus Under Dynamic Participation with a Well-Behaved Majority}},
  booktitle =	{37th International Symposium on Distributed Computing (DISC 2023)},
  pages =	{41:1--41:7},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-301-0},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{281},
  editor =	{Oshman, Rotem},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2023.41},
  URN =		{urn:nbn:de:0030-drops-191675},
  doi =		{10.4230/LIPIcs.DISC.2023.41},
  annote =	{Keywords: Consensus, Sleepy Model, Dynamic Participation, Byzantine Failures}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2022.17
Quorum Systems in Permissionless Networks

Authors: Christian Cachin, Giuliano Losa, and Luca Zanolini

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

Abstract
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.
Cite as

Christian Cachin, Giuliano Losa, and Luca Zanolini. Quorum Systems in Permissionless Networks. In 26th International Conference on Principles of Distributed Systems (OPODIS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 253, pp. 17:1-17:22, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2023)

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@InProceedings{cachin_et_al:LIPIcs.OPODIS.2022.17,
  author =	{Cachin, Christian and Losa, Giuliano and Zanolini, Luca},
  title =	{{Quorum Systems in Permissionless Networks}},
  booktitle =	{26th International Conference on Principles of Distributed Systems (OPODIS 2022)},
  pages =	{17:1--17:22},
  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.2022.17},
  URN =		{urn:nbn:de:0030-drops-176379},
  doi =		{10.4230/LIPIcs.OPODIS.2022.17},
  annote =	{Keywords: Permissionless systems, fail-prone system, quorum system}
}
Document
DOI: 10.4230/LIPIcs.DISC.2022.12
Liveness and Latency of Byzantine State-Machine Replication

Authors: Manuel Bravo, Gregory Chockler, and Alexey Gotsman

Published in: LIPIcs, Volume 246, 36th International Symposium on Distributed Computing (DISC 2022)

Abstract
Byzantine state-machine replication (SMR) ensures the consistency of replicated state in the presence of malicious replicas and lies at the heart of the modern blockchain technology. Byzantine SMR protocols often guarantee safety under all circumstances and liveness only under synchrony. However, guaranteeing liveness even under this assumption is nontrivial. So far we have lacked systematic ways of incorporating liveness mechanisms into Byzantine SMR protocols, which often led to subtle bugs. To close this gap, we introduce a modular framework to facilitate the design of provably live and efficient Byzantine SMR protocols. Our framework relies on a view abstraction generated by a special SMR synchronizer primitive to drive the agreement on command ordering. We present a simple formal specification of an SMR synchronizer and its bounded-space implementation under partial synchrony. We also apply our specification to prove liveness and analyze the latency of three Byzantine SMR protocols via a uniform methodology. In particular, one of these results yields what we believe is the first rigorous liveness proof for the algorithmic core of the seminal PBFT protocol.
Cite as

Manuel Bravo, Gregory Chockler, and Alexey Gotsman. Liveness and Latency of Byzantine State-Machine Replication. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 12:1-12:19, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2022)

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@InProceedings{bravo_et_al:LIPIcs.DISC.2022.12,
  author =	{Bravo, Manuel and Chockler, Gregory and Gotsman, Alexey},
  title =	{{Liveness and Latency of Byzantine State-Machine Replication}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{12:1--12:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.12},
  URN =		{urn:nbn:de:0030-drops-172037},
  doi =		{10.4230/LIPIcs.DISC.2022.12},
  annote =	{Keywords: Replication, blockchain, partial synchrony, liveness}
}
Document
DOI: 10.4230/OASIcs.FMBC.2020.9
On the Formal Verification of the Stellar Consensus Protocol

Authors: Giuliano Losa and Mike Dodds

Published in: OASIcs, Volume 84, 2nd Workshop on Formal Methods for Blockchains (FMBC 2020)

Abstract
The Stellar Consensus Protocol (SCP) is a quorum-based BFT consensus protocol. However, instead of using threshold-based quorums, SCP is permissionless and its quorum system emerges from participants’ self-declared trust relationships. In this paper, we describe the methodology we deploy to formally verify the safety and liveness of SCP for arbitrary but fixed configurations. The proof uses a combination of Ivy and Isabelle/HOL. In Ivy, we model SCP in first-order logic, and we verify safety and liveness under eventual synchrony. In Isabelle/HOL, we prove the validity of our first-order encoding with respect to a more direct higher-order model. SCP is currently deployed in the Stellar Network, and we believe this is the first mechanized proof of both safety and liveness, specified in LTL, for a deployed BFT protocol.
Cite as

Giuliano Losa and Mike Dodds. On the Formal Verification of the Stellar Consensus Protocol. In 2nd Workshop on Formal Methods for Blockchains (FMBC 2020). Open Access Series in Informatics (OASIcs), Volume 84, pp. 9:1-9:9, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2020)

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@InProceedings{losa_et_al:OASIcs.FMBC.2020.9,
  author =	{Losa, Giuliano and Dodds, Mike},
  title =	{{On the Formal Verification of the Stellar Consensus Protocol}},
  booktitle =	{2nd Workshop on Formal Methods for Blockchains (FMBC 2020)},
  pages =	{9:1--9:9},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-169-6},
  ISSN =	{2190-6807},
  year =	{2020},
  volume =	{84},
  editor =	{Bernardo, Bruno and Marmsoler, Diego},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.FMBC.2020.9},
  URN =		{urn:nbn:de:0030-drops-134226},
  doi =		{10.4230/OASIcs.FMBC.2020.9},
  annote =	{Keywords: Consensus, Blockchains, First-Order Logic, Stellar, Ivy Prover, Decidability}
}
Document
DOI: 10.4230/LIPIcs.DISC.2020.23
Making Byzantine Consensus Live

Authors: Manuel Bravo, Gregory Chockler, and Alexey Gotsman

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

Abstract
Partially synchronous Byzantine consensus protocols typically structure their execution into a sequence of views, each with a designated leader process. The key to guaranteeing liveness in these protocols is to ensure that all correct processes eventually overlap in a view with a correct leader for long enough to reach a decision. We propose a simple view synchronizer abstraction that encapsulates the corresponding functionality for Byzantine consensus protocols, thus simplifying their design. We present a formal specification of a view synchronizer and its implementation under partial synchrony, which runs in bounded space despite tolerating message loss during asynchronous periods. We show that our synchronizer specification is strong enough to guarantee liveness for single-shot versions of several well-known Byzantine consensus protocols, including HotStuff, Tendermint, PBFT and SBFT. We furthermore give precise latency bounds for these protocols when using our synchronizer. By factoring out the functionality of view synchronization we are able to specify and analyze the protocols in a uniform framework, which allows comparing them and highlights trade-offs.
Cite as

Manuel Bravo, Gregory Chockler, and Alexey Gotsman. Making Byzantine Consensus Live. In 34th International Symposium on Distributed Computing (DISC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 179, pp. 23:1-23:17, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2020)

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@InProceedings{bravo_et_al:LIPIcs.DISC.2020.23,
  author =	{Bravo, Manuel and Chockler, Gregory and Gotsman, Alexey},
  title =	{{Making Byzantine Consensus Live}},
  booktitle =	{34th International Symposium on Distributed Computing (DISC 2020)},
  pages =	{23:1--23:17},
  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.2020.23},
  URN =		{urn:nbn:de:0030-drops-131013},
  doi =		{10.4230/LIPIcs.DISC.2020.23},
  annote =	{Keywords: Byzantine consensus, blockchain, partial synchrony, liveness}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2019.5
Deconstructing Stellar Consensus

Authors: Álvaro García-Pérez and Maria A. Schett

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

Abstract
Some of the recent blockchain proposals, such as Stellar and Ripple, allow for open membership while using quorum-like structures typical for classical Byzantine consensus with closed membership. This is achieved by constructing quorums in a decentralised way: each participant independently chooses whom to trust, and quorums arise from these individual decisions. Unfortunately, the consensus protocols underlying such blockchains are poorly understood, and their correctness has not been rigorously investigated. In this paper we rigorously prove correct the Stellar Consensus Protocol (SCP), with our proof giving insights into the protocol structure and its use of lower-level abstractions. To this end, we first propose an abstract version of SCP that uses as a black box Stellar’s federated voting primitive (analogous to reliable Byzantine broadcast), previously investigated by García-Pérez and Gotsman [Álvaro García-Pérez and Alexey Gotsman, 2018]. The abstract consensus protocol highlights a modular structure in Stellar and can be proved correct by reusing the previous results on federated voting. However, it is unsuited for realistic implementations, since its processes maintain infinite state. We thus establish a refinement between the abstract protocol and the concrete SCP that uses only finite state, thereby carrying over the result about the correctness of former to the latter. Our results help establish the theoretical foundations of decentralised blockchains like Stellar and gain confidence in their correctness.
Cite as

Álvaro García-Pérez and Maria A. Schett. Deconstructing Stellar Consensus. In 23rd International Conference on Principles of Distributed Systems (OPODIS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 153, pp. 5:1-5:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2020)

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@InProceedings{garciaperez_et_al:LIPIcs.OPODIS.2019.5,
  author =	{Garc{\'\i}a-P\'{e}rez, \'{A}lvaro and Schett, Maria A.},
  title =	{{Deconstructing Stellar Consensus}},
  booktitle =	{23rd International Conference on Principles of Distributed Systems (OPODIS 2019)},
  pages =	{5:1--5:16},
  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.2019.5},
  URN =		{urn:nbn:de:0030-drops-117910},
  doi =		{10.4230/LIPIcs.OPODIS.2019.5},
  annote =	{Keywords: Blockchain, Consensus protocol, Stellar, Byzantine quorum systems}
}
Document
DOI: 10.4230/LIPIcs.DISC.2019.27
Stellar Consensus by Instantiation

Authors: Giuliano Losa, Eli Gafni, and David Mazières

Published in: LIPIcs, Volume 146, 33rd International Symposium on Distributed Computing (DISC 2019)

Abstract
Stellar introduced a new type of quorum system called a Federated Byzantine Agreement System. A major difference between this novel type of quorum system and a threshold quorum system is that each participant has its own, personal notion of a quorum. Thus, unlike in a traditional BFT system, designed for a uniform notion of quorum, even in a time of synchrony one well-behaved participant may observe a quorum of well-behaved participants, while others may not. To tackle this new problem in a more general setting, we abstract the Stellar Network as an instance of what we call Personal Byzantine Quorum Systems. Using this notion, we streamline the theory behind the Stellar Network, removing the clutter of unnecessary details, and refute the conjecture that Stellar’s notion of intact set is optimally fault-tolerant. Most importantly, we develop a new consensus algorithm for the new setting.
Cite as

Giuliano Losa, Eli Gafni, and David Mazières. Stellar Consensus by Instantiation. In 33rd International Symposium on Distributed Computing (DISC 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 146, pp. 27:1-27:15, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2019)

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@InProceedings{losa_et_al:LIPIcs.DISC.2019.27,
  author =	{Losa, Giuliano and Gafni, Eli and Mazi\`{e}res, David},
  title =	{{Stellar Consensus by Instantiation}},
  booktitle =	{33rd International Symposium on Distributed Computing (DISC 2019)},
  pages =	{27:1--27:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-126-9},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{146},
  editor =	{Suomela, Jukka},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2019.27},
  URN =		{urn:nbn:de:0030-drops-113343},
  doi =		{10.4230/LIPIcs.DISC.2019.27},
  annote =	{Keywords: Consensus, Stellar, Partial Synchrony, Byzantine Fault Tolerance}
}
Document
DOI: 10.4230/LIPIcs.DISC.2018.17
A Wealth of Sub-Consensus Deterministic Objects

Authors: Eli Daian, Giuliano Losa, Yehuda Afek, and Eli Gafni

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

Abstract
The consensus hierarchy classifies shared an object according to its consensus number, which is the maximum number of processes that can solve consensus wait-free using the object. The question of whether this hierarchy is precise enough to fully characterize the synchronization power of deterministic shared objects was open until 2016, when Afek et al. showed that there is an infinite hierarchy of deterministic objects, each weaker than the next, which is strictly between i and i+1-processors consensus, for i >= 2. For i=1, the question whether there exist a deterministic object whose power is strictly between read-write and 2-processors consensus, remained open. We resolve the question positively by exhibiting an infinite hierarchy of simple deterministic objects which are equivalent to set-consensus tasks, and thus are stronger than read-write registers, but they cannot implement consensus for two processes. Still our paper leaves a gap with open questions.
Cite as

Eli Daian, Giuliano Losa, Yehuda Afek, and Eli Gafni. A Wealth of Sub-Consensus Deterministic Objects. In 32nd International Symposium on Distributed Computing (DISC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 121, pp. 17:1-17:17, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2018)

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@InProceedings{daian_et_al:LIPIcs.DISC.2018.17,
  author =	{Daian, Eli and Losa, Giuliano and Afek, Yehuda and Gafni, Eli},
  title =	{{A Wealth of Sub-Consensus Deterministic Objects}},
  booktitle =	{32nd International Symposium on Distributed Computing (DISC 2018)},
  pages =	{17:1--17:17},
  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.2018.17},
  URN =		{urn:nbn:de:0030-drops-98061},
  doi =		{10.4230/LIPIcs.DISC.2018.17},
  annote =	{Keywords: shared memory, distributed algorithms, wait-free, set consensus}
}
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