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Documents authored by Losa, Giuliano

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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/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.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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