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Documents authored by Widder, Josef

Document
DOI: 10.4230/LIPIcs.DISC.2022.10
Holistic Verification of Blockchain Consensus

Authors: Nathalie Bertrand, Vincent Gramoli, Igor Konnov, Marijana Lazić, Pierre Tholoniat, and Josef Widder

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

Abstract
Blockchain has recently attracted the attention of the industry due, in part, to its ability to automate asset transfers. It requires distributed participants to reach a consensus on a block despite the presence of malicious (a.k.a. Byzantine) participants. Malicious participants exploit regularly weaknesses of these blockchain consensus algorithms, with sometimes devastating consequences. In fact, these weaknesses are quite common and are well illustrated by the flaws in various blockchain consensus algorithms [Pierre Tholoniat and Vincent Gramoli, 2019]. Paradoxically, until now, no blockchain consensus has been holistically verified. In this paper, we remedy this paradox by model checking for the first time a blockchain consensus used in industry. We propose a holistic approach to verify the consensus algorithm of the Red Belly Blockchain [Tyler Crain et al., 2021], for any number n of processes and any number f < n/3 of Byzantine processes. We decompose directly the algorithm pseudocode in two parts - an inner broadcast algorithm and an outer decision algorithm - each modelled as a threshold automaton [Igor Konnov et al., 2017], and we formalize their expected properties in linear-time temporal logic. We then automatically check the inner broadcasting algorithm, under a carefully identified fairness assumption. For the verification of the outer algorithm, we simplify the model of the inner algorithm by relying on its proven properties. Doing so, we formally verify, for any parameter, not only the safety properties of the Red Belly Blockchain consensus but also its liveness in less than 70 seconds.
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Nathalie Bertrand, Vincent Gramoli, Igor Konnov, Marijana Lazić, Pierre Tholoniat, and Josef Widder. Holistic Verification of Blockchain Consensus. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 10:1-10:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{bertrand_et_al:LIPIcs.DISC.2022.10,
  author =	{Bertrand, Nathalie and Gramoli, Vincent and Konnov, Igor and Lazi\'{c}, Marijana and Tholoniat, Pierre and Widder, Josef},
  title =	{{Holistic Verification of Blockchain Consensus}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{10:1--10:24},
  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.2022.10},
  URN =		{urn:nbn:de:0030-drops-172019},
  doi =		{10.4230/LIPIcs.DISC.2022.10},
  annote =	{Keywords: Model checking, automata, logic, byzantine failure}
}
Document
DOI: 10.4230/LIPIcs.CONCUR.2021.15
Guard Automata for the Verification of Safety and Liveness of Distributed Algorithms

Authors: Nathalie Bertrand, Bastien Thomas, and Josef Widder

Published in: LIPIcs, Volume 203, 32nd International Conference on Concurrency Theory (CONCUR 2021)

Abstract
Distributed algorithms typically run over arbitrary many processes and may involve unboundedly many rounds, making the automated verification of their correctness challenging. Building on domain theory, we introduce a framework that abstracts infinite-state distributed systems that represent distributed algorithms into finite-state guard automata. The soundness of the approach corresponds to the Scott-continuity of the abstraction, which relies on the assumption that the distributed algorithms are layered. Guard automata thus enable the verification of safety and liveness properties of distributed algorithms.
Cite as

Nathalie Bertrand, Bastien Thomas, and Josef Widder. Guard Automata for the Verification of Safety and Liveness of Distributed Algorithms. In 32nd International Conference on Concurrency Theory (CONCUR 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 203, pp. 15:1-15:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{bertrand_et_al:LIPIcs.CONCUR.2021.15,
  author =	{Bertrand, Nathalie and Thomas, Bastien and Widder, Josef},
  title =	{{Guard Automata for the Verification of Safety and Liveness of Distributed Algorithms}},
  booktitle =	{32nd International Conference on Concurrency Theory (CONCUR 2021)},
  pages =	{15:1--15:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-203-7},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{203},
  editor =	{Haddad, Serge and Varacca, Daniele},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2021.15},
  URN =		{urn:nbn:de:0030-drops-143926},
  doi =		{10.4230/LIPIcs.CONCUR.2021.15},
  annote =	{Keywords: Verification, Distributed algorithms, Domain theory}
}
Document
Short Paper
DOI: 10.4230/OASIcs.FMBC.2020.10
Formal Specification and Model Checking of the Tendermint Blockchain Synchronization Protocol (Short Paper)

Authors: Sean Braithwaite, Ethan Buchman, Igor Konnov, Zarko Milosevic, Ilina Stoilkovska, Josef Widder, and Anca Zamfir

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

Abstract
Blockchain synchronization is one of the core protocols of Tendermint blockchains. In this short paper, we discuss our recent efforts in formal specification of the protocol and its implementation, as well as some initial model checking results. We demonstrate that the protocol quality and understanding can be improved by writing specifications and model checking them.
Cite as

Sean Braithwaite, Ethan Buchman, Igor Konnov, Zarko Milosevic, Ilina Stoilkovska, Josef Widder, and Anca Zamfir. Formal Specification and Model Checking of the Tendermint Blockchain Synchronization Protocol (Short Paper). In 2nd Workshop on Formal Methods for Blockchains (FMBC 2020). Open Access Series in Informatics (OASIcs), Volume 84, pp. 10:1-10:8, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{braithwaite_et_al:OASIcs.FMBC.2020.10,
  author =	{Braithwaite, Sean and Buchman, Ethan and Konnov, Igor and Milosevic, Zarko and Stoilkovska, Ilina and Widder, Josef and Zamfir, Anca},
  title =	{{Formal Specification and Model Checking of the Tendermint Blockchain Synchronization Protocol}},
  booktitle =	{2nd Workshop on Formal Methods for Blockchains (FMBC 2020)},
  pages =	{10:1--10:8},
  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.2020.10},
  URN =		{urn:nbn:de:0030-drops-134238},
  doi =		{10.4230/OASIcs.FMBC.2020.10},
  annote =	{Keywords: Blockchain, Fault Tolerance, Byzantine Faults, Model Checking}
}
Document
DOI: 10.4230/LIPIcs.CONCUR.2019.33
Verification of Randomized Consensus Algorithms Under Round-Rigid Adversaries

Authors: Nathalie Bertrand, Igor Konnov, Marijana Lazić, and Josef Widder

Published in: LIPIcs, Volume 140, 30th International Conference on Concurrency Theory (CONCUR 2019)

Abstract
Randomized fault-tolerant distributed algorithms pose a number of challenges for automated verification: (i) parameterization in the number of processes and faults, (ii) randomized choices and probabilistic properties, and (iii) an unbounded number of asynchronous rounds. This combination makes verification hard. Challenge (i) was recently addressed in the framework of threshold automata. We extend threshold automata to model randomized consensus algorithms that perform an unbounded number of asynchronous rounds. For non-probabilistic properties, we show that it is necessary and sufficient to verify these properties under round-rigid schedules, that is, schedules where processes enter round r only after all processes finished round r-1. For almost-sure termination, we analyze these algorithms under round-rigid adversaries, that is, fair adversaries that only generate round-rigid schedules. This allows us to do compositional and inductive reasoning that reduces verification of the asynchronous multi-round algorithms to model checking of a one-round threshold automaton. We apply this framework and automatically verify the following classic algorithms: Ben-Or’s and Bracha’s seminal consensus algorithms for crashes and Byzantine faults, 2-set agreement for crash faults, and RS-Bosco for the Byzantine case.
Cite as

Nathalie Bertrand, Igor Konnov, Marijana Lazić, and Josef Widder. Verification of Randomized Consensus Algorithms Under Round-Rigid Adversaries. In 30th International Conference on Concurrency Theory (CONCUR 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 140, pp. 33:1-33:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{bertrand_et_al:LIPIcs.CONCUR.2019.33,
  author =	{Bertrand, Nathalie and Konnov, Igor and Lazi\'{c}, Marijana and Widder, Josef},
  title =	{{Verification of Randomized Consensus Algorithms Under Round-Rigid Adversaries}},
  booktitle =	{30th International Conference on Concurrency Theory (CONCUR 2019)},
  pages =	{33:1--33:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-121-4},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{140},
  editor =	{Fokkink, Wan and van Glabbeek, Rob},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2019.33},
  URN =		{urn:nbn:de:0030-drops-109358},
  doi =		{10.4230/LIPIcs.CONCUR.2019.33},
  annote =	{Keywords: threshold automata, counter systems, parameterized verification, randomized distributed algorithms, Byzantine faults}
}
Document
Complete Volume
DOI: 10.4230/LIPIcs.DISC.2018
LIPIcs, Volume 121, DISC'18, Complete Volume

Authors: Ulrich Schmid and Josef Widder

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

Abstract
LIPIcs, Volume 121, DISC'18, Complete Volume
Cite as

32nd International Symposium on Distributed Computing (DISC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 121, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@Proceedings{schmid_et_al:LIPIcs.DISC.2018,
  title =	{{LIPIcs, Volume 121, DISC'18, Complete Volume}},
  booktitle =	{32nd International Symposium on Distributed Computing (DISC 2018)},
  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},
  URN =		{urn:nbn:de:0030-drops-98456},
  doi =		{10.4230/LIPIcs.DISC.2018},
  annote =	{Keywords: Software and its engineering, Distributed systems organizing principles, Computing methodologies, Distributed computing methodologies}
}
Document
Front Matter
DOI: 10.4230/LIPIcs.DISC.2018.0
Front Matter, Table of Contents, Preface, Conference Organization, Awards

Authors: Ulrich Schmid and Josef Widder

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

Abstract
Front Matter, Table of Contents, Preface, Conference Organization, Awards
Cite as

32nd International Symposium on Distributed Computing (DISC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 121, pp. 0:i-0:xx, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{schmid_et_al:LIPIcs.DISC.2018.0,
  author =	{Schmid, Ulrich and Widder, Josef},
  title =	{{Front Matter, Table of Contents, Preface, Conference Organization, Awards}},
  booktitle =	{32nd International Symposium on Distributed Computing (DISC 2018)},
  pages =	{0:i--0:xx},
  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.2018.0},
  URN =		{urn:nbn:de:0030-drops-97899},
  doi =		{10.4230/LIPIcs.DISC.2018.0},
  annote =	{Keywords: Front Matter, Table of Contents, Preface, Conference Organization, Awards}
}
Document
DOI: 10.4230/LIPIcs.CONCUR.2018.19
Reachability in Parameterized Systems: All Flavors of Threshold Automata

Authors: Jure Kukovec, Igor Konnov, and Josef Widder

Published in: LIPIcs, Volume 118, 29th International Conference on Concurrency Theory (CONCUR 2018)

Abstract
Threshold automata, and the counter systems they define, were introduced as a framework for parameterized model checking of fault-tolerant distributed algorithms. This application domain suggested natural constraints on the automata structure, and a specific form of acceleration, called single-rule acceleration: consecutive occurrences of the same automaton rule are executed as a single transition in the counter system. These accelerated systems have bounded diameter, and can be verified in a complete manner with bounded model checking. We go beyond the original domain, and investigate extensions of threshold automata: non-linear guards, increments and decrements of shared variables, increments of shared variables within loops, etc., and show that the bounded diameter property holds for several extensions. Finally, we put single-rule acceleration in the scope of flat counter automata: although increments in loops may break the bounded diameter property, the corresponding counter automaton is flattable, and reachability can be verified using more permissive forms of acceleration.
Cite as

Jure Kukovec, Igor Konnov, and Josef Widder. Reachability in Parameterized Systems: All Flavors of Threshold Automata. In 29th International Conference on Concurrency Theory (CONCUR 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 118, pp. 19:1-19:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{kukovec_et_al:LIPIcs.CONCUR.2018.19,
  author =	{Kukovec, Jure and Konnov, Igor and Widder, Josef},
  title =	{{Reachability in Parameterized Systems: All Flavors of Threshold Automata}},
  booktitle =	{29th International Conference on Concurrency Theory (CONCUR 2018)},
  pages =	{19:1--19:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-087-3},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{118},
  editor =	{Schewe, Sven and Zhang, Lijun},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2018.19},
  URN =		{urn:nbn:de:0030-drops-95578},
  doi =		{10.4230/LIPIcs.CONCUR.2018.19},
  annote =	{Keywords: threshold \& counter automata, parameterized verification, reachability}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2017.32
Synthesis of Distributed Algorithms with Parameterized Threshold Guards

Authors: Marijana Lazic, Igor Konnov, Josef Widder, and Roderick Bloem

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

Abstract
Fault-tolerant distributed algorithms are notoriously hard to get right. In this paper we introduce an automated method that helps in that process: the designer provides specifications (the problem to be solved) and a sketch of a distributed algorithm that keeps arithmetic details unspecified. Our tool then automatically fills the missing parts. Fault-tolerant distributed algorithms are typically parameterized, that is, they are designed to work for any number n of processes and any number t of faults, provided some resilience condition holds; e.g., n > 3t. In this paper we automatically synthesize distributed algorithms that work for all parameter values that satisfy the resilience condition. We focus on threshold- guarded distributed algorithms, where actions are taken only if a sufficiently large number of messages is received, e.g., more than t or n/2. Both expressions can be derived by choosing the right values for the coefficients a, b, and c, in the sketch of a threshold a·n+b·t+c. Our method takes as input a sketch of an asynchronous threshold-based fault-tolerant distributed algorithm — where the guards are missing exact coefficients—and then iteratively picks the values for the coefficients. Our approach combines recent progress in parameterized model checking of distributed algo- rithms with counterexample-guided synthesis. Besides theoretical results on termination of the synthesis procedure, we experimentally evaluate our method and show that it can synthesize sev- eral distributed algorithms from the literature, e.g., Byzantine reliable broadcast and Byzantine one-step consensus. In addition, for several new variations of safety and liveness specifications, our tool generates new distributed algorithms.
Cite as

Marijana Lazic, Igor Konnov, Josef Widder, and Roderick Bloem. Synthesis of Distributed Algorithms with Parameterized Threshold Guards. In 21st International Conference on Principles of Distributed Systems (OPODIS 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 95, pp. 32:1-32:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{lazic_et_al:LIPIcs.OPODIS.2017.32,
  author =	{Lazic, Marijana and Konnov, Igor and Widder, Josef and Bloem, Roderick},
  title =	{{Synthesis of Distributed Algorithms with Parameterized Threshold Guards}},
  booktitle =	{21st International Conference on Principles of Distributed Systems (OPODIS 2017)},
  pages =	{32:1--32:20},
  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.2017.32},
  URN =		{urn:nbn:de:0030-drops-86359},
  doi =		{10.4230/LIPIcs.OPODIS.2017.32},
  annote =	{Keywords: fault-tolerant distributed algorithms, byzantine faults, parameterized model checking, program synthesis}
}
Document
DOI: 10.4230/DagRep.3.4.1
Formal Verification of Distributed Algorithms (Dagstuhl Seminar 13141)

Authors: Bernadette Charron-Bost, Stephan Merz, Andrey Rybalchenko, and Josef Widder

Published in: Dagstuhl Reports, Volume 3, Issue 4 (2013)

Abstract
The Dagstuhl Seminar 13141 "Formal Verification of Distributed Algorithms" brought together researchers from the areas of distributed algorithms, model checking, and semi-automated proofs with the goal to establish a common base for approaching the many open problems in verification of distributed algorithms. In order to tighten the gap between the involved communities, who have been quite separated in the past, the program contained tutorials on the basics of the concerned fields. In addition to technical talks, we also had several discussion sessions, whose goal was to identify the most pressing research challenges. This report describes the program and the outcomes of the seminar.
Cite as

Bernadette Charron-Bost, Stephan Merz, Andrey Rybalchenko, and Josef Widder. Formal Verification of Distributed Algorithms (Dagstuhl Seminar 13141). In Dagstuhl Reports, Volume 3, Issue 4, pp. 1-16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2013)

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@Article{charronbost_et_al:DagRep.3.4.1,
  author =	{Charron-Bost, Bernadette and Merz, Stephan and Rybalchenko, Andrey and Widder, Josef},
  title =	{{Formal Verification of Distributed Algorithms (Dagstuhl Seminar 13141)}},
  pages =	{1--16},
  journal =	{Dagstuhl Reports},
  ISSN =	{2192-5283},
  year =	{2013},
  volume =	{3},
  number =	{4},
  editor =	{Charron-Bost, Bernadette and Merz, Stephan and Rybalchenko, Andrey and Widder, Josef},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DagRep.3.4.1},
  URN =		{urn:nbn:de:0030-drops-40747},
  doi =		{10.4230/DagRep.3.4.1},
  annote =	{Keywords: Distributed algorithms; semi-automated proofs; model checking}
}
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