Self-Stabilizing Byzantine-Resilient Communication in Dynamic Networks

We consider the problem of communicating reliably in a dynamic network in the presence of up to k Byzantine failures. It was shown that this problem can be solved if and only if the dynamic graph satisfies a certain condition, that we call "RDC condition". In this paper, we present the first self-stabilizing algorithm for reliable communication in this setting - that is: in addition to permanent Byzantine failures, there can also be an arbitrary number of transient failures. We prove the correctness of this algorithm, provided that the RDC condition is "always eventually satisfied"

Multi-hop Byzantine reliable broadcast with honest dealer made practical

We revisit Byzantine tolerant reliable broadcast with honest dealer algorithms in multi-hop networks. To tolerate Byzantine faulty nodes arbitrarily spread over the network, previous solutions require a factorial number of messages to be sent over the network if the messages are not authenticated (e.g., digital signatures are not available). We propose modifications that preserve the safety and liveness properties of the original unauthenticated protocols, while highly decreasing their observed message complexity when simulated on several classes of graph topologies, potentially opening to their employment

Multi-hop Byzantine Reliable Broadcast with Honest Dealer Made Practical

We revisit Byzantine tolerant reliable broadcast with honest dealer algorithms in multi-hop networks. To tolerate Byzantine faulty nodes arbitrarily spread over the network, previous solutions require a factorial number of messages to be sent over the network if the messages are not authenticated (e.g. digital signatures are not available). We propose modifications that preserve the safety and liveness properties of the original unauthenticated protocols, while highly decreasing their observed message complexity when simulated on several classes of graph topologies, potentially opening to their employment

Multi-hop Byzantine Reliable Broadcast with Honest Dealer Made Practical

We revisit Byzantine tolerant reliable broadcast with honest dealer algorithms in multi-hop networks. To tolerate Byzantine faulty nodes arbitrarily spread over the network, previous solutions require a factorial number of messages to be sent over the network if the messages are not authenticated (e.g. digital signatures are not available). We propose modifications that preserve the safety and liveness properties of the original unauthenticated protocols, while highly decreasing their observed message complexity when simulated on several classes of graph topologies, potentially opening to their employment

Byzantine fault-tolerant vote collection for D-DEMOS, a distributed e-voting system

Τα συστήματα διαχείρισης εκλογών είναι μια δυναμική τεχνολογία που επιτρέπει την βελτίωση της δημοκρατικής διαδικασίας μέσω της μείωσης του κόστους υλοποίησης εκλογών, της αύξησης της συμμετοχής των ψηφοφόρων και της αμεσότητας παραγωγής αποτελεσμάτων. Επίσης, δίνουν την δυνατότητα στους ψηφοφόρους να επιβεβαιώσουν άμεσα την ορθή λειτουργία ολόκληρης της εκλογικής διαδικασίας. Δυστυχώς, τα υπάρχοντα τέτοια συστήματα είναι σχεδιασμένα με κεντρικά συστατικά, τα οποία και αποτελούν μοναδικά σημεία αποτυχίας. Αυτό μπορεί να οδηγήσει στην απώλεια διαθεσιμότητας, εμπιστευτικότητας, καθώς και της ακεραιότητας του εκλογικού αποτελέσματος. Σε αυτή τη διατριβή εξετάζουμε την εισαγωγή ανοχής λαθών στα εκλογικά συστήματα, μέσω της εισαγωγής κατανεμημένων συστατικών. Αυτό είναι περίπλοκο γιατί, εκτός από την ακεραιότητα και διαθεσιμότητα, σε ένα εκλογικό σύστημα είναι σημαντικό να διαφυλαχθεί και η εμπιστευτικότητα, απέναντι σε έναν κακόβουλο αντίπαλο. Εστιάζουμε στην φάση συλλογής ψήφων του εκλογικού συστήματος, η οποία είναι ένα κρίσιμο τμήμα της εκλογικής διαδικασίας. Χρησιμοποιούμε το σύγχρονο αλλά κεντρικοποιημένο σύστημα διαχείρισης εκλογών DEMOS σαν βάση για την μελέτη μας. Αυτό το σύστημα χρησιμοποιεί κωδικούς που αντιστοιχούν στις δυνατές επιλογές των ψηφοφόρων, μια Αρχή Εκλογών η οποία αρχικοποιεί τις εκλογές, συλλέγει τις ψήφους και παράγει το αποτέλεσμα, και έναν Πίνακα Ανακοινώσεων για την διατήρηση των στοιχείων των εκλογών μακροπρόθεσμα. Εξάγουμε τον μηχανισμό συλλογής ψήφων από την κεντρικοποιημένη Αρχή Εκλογών του αρχικού συστήματος DEMOS, και τον αντικαθιστούμε με ένα κατανεμημένο σύστημα που χειρίζεται την συλλογή ψήφων με ανοχή σε λάθη Βυζαντινού τύπου. Σε αυτή τη διατριβή, παρουσιάζουμε τον σχεδιασμό, ανάλυση ασφάλειας, την ανάπτυξη και αξιολόγηση της πρωτότυπης υλοποίησης αυτού του κατανεμημένου συστατικού συλλογής ψήφων. Παρουσιάζουμε δύο εκδόσεις αυτού του συστατικού: μία πλήρως ασύγχρονη και μία με ελάχιστες υποθέσεις συγχρονισμού αλλά καλύτερη απόδοση. Και οι δύο εκδόσεις παρέχουν άμεση επιβεβαίωση στην ψηφοφόρο ότι η ψήφος της καταχωρήθηκε όπως υποβλήθηκε, χωρίς να απαιτούνται κρυπτογραφικές λειτουργίες από την πλευρά της ψηφοφόρου. Με αυτόν τον τρόπο, η ψηφοφόρος μπορεί να στείλει την ψήφο της χρησιμοποιώντας έναν μη ασφαλή υπολογιστή ή δίκτυο, και να συνεχίσει να είναι εξασφαλισμένη ότι η ψήφος της καταχωρήθηκε σωστά. Για παράδειγμα, μπορεί να ψηφίσει χρησιμοποιώντας έναν δημόσιο υπολογιστή, ή στέλνοντας ένα σύντομο μήνυμα μέσω κινητού τηλεφώνου. Ακόμη και σε αυτές τις περιπτώσεις, η εμπιστευτικότητα της ψήφου διατηρείται στο ακέραιο. Δίνουμε ένα μοντέλο και μια ανάλυση ασφάλειας για τα συστήματα που παρουσιάζουμε. Υλοποιούμε πρωτότυπα από τα πλήρη συστήματα, μετράμε την απόδοσή τους πειραματικά, και επιδεικνύουμε την ικανότητά τους να χειρίζονται εκλογές μεγάλου μεγέθους. Τέλος, παρουσιάζουμε τις διαφορές απόδοσης ανάμεσα στις δύο εκδόσεις του συστήματος. Θεωρούμε ότι τα συστατικά συλλογής ψήφων που παρουσιάζουμε σε αυτή τη διατριβή μπορούν να βρουν εφαρμογή σε οποιοδήποτε σύστημα διαχείρισης εκλογών που στηρίζεται στην τεχνική της εκπροσώπησης των επιλογών στα ψηφοδέλτια με κωδικούς.E-voting systems are a powerful technology for improving democracy by reducing election cost, increasing voter participation, and even allowing voters to directly verify the entire election procedure. Unfortunately, prior internet voting systems have single points of failure, which may result in the compromise of availability, voter secrecy, or integrity of the election results. In this thesis, we consider increasing the fault-tolerance of voting systems by introducing distributed components. This is non-trivial as, besides integrity and availability, voting requires safeguarding confidentiality as well, against a malicious adversary. We focus on the vote collection phase of the voting system, which is a crucial part of the election process. We use the DEMOS state-of-the-art but centralized voting system as the basis for our study. This system uses vote codes to represent voters' choices, an Election Authority to setup the election and handle vote collection and result production, and a Bulletin Board for storing the election transcript for the long-term. We extract the vote collection mechanism from the centralized Election Authority component of the original DEMOS system, and replace it with a distributed system that handles vote collection in a Byzantine fault-tolerant manner. In this thesis, we present the design, security analysis, prototype implementation and experimental evaluation of this vote collection component. We present two versions of this component: one completely asynchronous and one with minimal timing assumptions but better performance. Both versions provide immediate assurance to the voter her vote was recorded as cast, without requiring cryptographic operations on behalf of the voter. This way, a voter may cast her vote using an untrusted computer or network, and still be assured her vote was recorded as cast. For example, she may vote via a public web terminal, or by sending an SMS from a mobile phone. Even in these cases, voter's privacy is still preserved. We provide a model and security analysis of the systems we present. We implement prototypes of the complete systems, we measure their performance experimentally, and we demonstrate their ability to handle large-scale elections. Finally, we demonstrate the performance trade-offs between the two versions of the system. We consider the vote collection components we introduce are applicable to any voting system that uses the code-voting technique

A Practical Analysis of the Gorums Framework: A Case Study on Replicated Services with Raft

Master's thesis in Computer scienceGorums is a novel RPC framework developed to make it easier to build fault tolerant distributed systems. We want to assess whether Gorums can simplify the implementation of a practical fault tolerant service that supports reconfiguration. The Raft consensus algorithm is implemented in Gorums with the ability to do single-server configuration changes. In addition, we perform a background study of two state of the art Raft implementations. The abstractions used in these implementations are then compared to the abstractions Gorums provides and how they are used in our Raft implementation. A service is created that can be used with any of the aforementioned Raft implementations for consistency and fault tolerance. This service is then used to evaluate the different implementations through experimentation. Our evaluation shows that the Raft implementation that uses Gorums perform better with regards to latency and overall throughput during normal operation. We do however discover this implementation to be sensitive to omission faults, which can further lead to availability issues if not handled properly. We solve this by developing extensions to Raft and Gorums. We show that these methods perform on a similar level when compared with the state of the art implementations. Results from our implementation efforts indicate that Raft's log replication process is problematic to implement with Gorums' abstractions. We discover that this is due to Raft adopting a monolithic design aimed to reduce the number of different RPC types, breaching the separation of concerns design principle

Políticas de Copyright de Publicações Científicas em Repositórios Institucionais: O Caso do INESC TEC

A progressiva transformação das práticas científicas, impulsionada pelo desenvolvimento das novas Tecnologias de Informação e Comunicação (TIC), têm possibilitado aumentar o acesso à informação, caminhando gradualmente para uma abertura do ciclo de pesquisa. Isto permitirá resolver a longo prazo uma adversidade que se tem colocado aos investigadores, que passa pela existência de barreiras que limitam as condições de acesso, sejam estas geográficas ou financeiras. Apesar da produção científica ser dominada, maioritariamente, por grandes editoras comerciais, estando sujeita às regras por estas impostas, o Movimento do Acesso Aberto cuja primeira declaração pública, a Declaração de Budapeste (BOAI), é de 2002, vem propor alterações significativas que beneficiam os autores e os leitores. Este Movimento vem a ganhar importância em Portugal desde 2003, com a constituição do primeiro repositório institucional a nível nacional. Os repositórios institucionais surgiram como uma ferramenta de divulgação da produção científica de uma instituição, com o intuito de permitir abrir aos resultados da investigação, quer antes da publicação e do próprio processo de arbitragem (preprint), quer depois (postprint), e, consequentemente, aumentar a visibilidade do trabalho desenvolvido por um investigador e a respetiva instituição. O estudo apresentado, que passou por uma análise das políticas de copyright das publicações científicas mais relevantes do INESC TEC, permitiu não só perceber que as editoras adotam cada vez mais políticas que possibilitam o auto-arquivo das publicações em repositórios institucionais, como também que existe todo um trabalho de sensibilização a percorrer, não só para os investigadores, como para a instituição e toda a sociedade. A produção de um conjunto de recomendações, que passam pela implementação de uma política institucional que incentive o auto-arquivo das publicações desenvolvidas no âmbito institucional no repositório, serve como mote para uma maior valorização da produção científica do INESC TEC.The progressive transformation of scientific practices, driven by the development of new Information and Communication Technologies (ICT), which made it possible to increase access to information, gradually moving towards an opening of the research cycle. This opening makes it possible to resolve, in the long term, the adversity that has been placed on researchers, which involves the existence of barriers that limit access conditions, whether geographical or financial. Although large commercial publishers predominantly dominate scientific production and subject it to the rules imposed by them, the Open Access movement whose first public declaration, the Budapest Declaration (BOAI), was in 2002, proposes significant changes that benefit the authors and the readers. This Movement has gained importance in Portugal since 2003, with the constitution of the first institutional repository at the national level. Institutional repositories have emerged as a tool for disseminating the scientific production of an institution to open the results of the research, both before publication and the preprint process and postprint, increase the visibility of work done by an investigator and his or her institution. The present study, which underwent an analysis of the copyright policies of INESC TEC most relevant scientific publications, allowed not only to realize that publishers are increasingly adopting policies that make it possible to self-archive publications in institutional repositories, all the work of raising awareness, not only for researchers but also for the institution and the whole society. The production of a set of recommendations, which go through the implementation of an institutional policy that encourages the self-archiving of the publications developed in the institutional scope in the repository, serves as a motto for a greater appreciation of the scientific production of INESC TEC

Computer Aided Verification

This open access two-volume set LNCS 13371 and 13372 constitutes the refereed proceedings of the 34rd International Conference on Computer Aided Verification, CAV 2022, which was held in Haifa, Israel, in August 2022. The 40 full papers presented together with 9 tool papers and 2 case studies were carefully reviewed and selected from 209 submissions. The papers were organized in the following topical sections: Part I: Invited papers; formal methods for probabilistic programs; formal methods for neural networks; software Verification and model checking; hyperproperties and security; formal methods for hardware, cyber-physical, and hybrid systems. Part II: Probabilistic techniques; automata and logic; deductive verification and decision procedures; machine learning; synthesis and concurrency. This is an open access book