Antonella Del Pozzo

Tight self-stabilizing mobile byzantine-tolerant atomic register

This paper proposes the first implementation of a self-stabilizing atomic register that is tolerant to both Mobile Byzantine Agents and transient failures. The register is maintained by n servers and our algorithm tolerates (i) any number of transient failures and (ii) up to f Mobile Byzantine Failures. In the Mobile Byzantine Failure model, faulty agents move from one server to another and when they are affecting a server, it behaves arbitrarily. Our implementation is designed for the round-based synchronous model where agents are moved from round to round. The paper considers four Mobile Byzantine Failure models differing for the diagnosis capabilities at server side i.e., when servers can diagnose their failure state (that is, be aware that the mobile Byzantine agent has left the server), and when servers cannot self-diagnose. We first prove lower bounds on the number of servers n necessary to construct a register tolerant to the presence of f Mobile Byzantine Failures for each of the Mobile Byzantine Failure models considered and then we propose a parametric algorithm working in all the models and matching the lower bounds.

Optimal self-stabilizing synchronous mobile Byzantine-tolerant atomic register

Abstract This paper addresses for the first time the problem of MWMR atomic memory in a Mobile Byzantine Agents model. The register is maintained by n servers and faulty (Byzantine) agents move from one server to another and when they are affecting a server, this one behaves arbitrarily. This paper addresses the round-based synchronous communication model. We focus on four Mobile Byzantine Failure models differing in the diagnosis capabilities at server side. We address the case when servers can diagnose their failure state (that is, servers are aware that the mobile Byzantine agent has left), and the case when servers cannot self-diagnose. We first prove lower bounds on the number of servers n necessary to construct a safe register tolerant to the presence of f n Mobile Byzantine Failures for four Mobile Byzantine Failure models. Additionally, we prove that our lower bounds do not change when the system is affected by any number of transient failures . Furthermore, we propose a parametric algorithm that implements an atomic MWMR register algorithm working in all the above models and matches the lower bounds. Additionally, our algorithm is also self-stabilizing. That is, started in an arbitrary state (i.e. after the occurrence of a transient failure) it is able to self-recover a correct behavior in a finite, bounded number of rounds. Our algorithm tolerates (i) any number of transient failures and (ii) up to f Mobile Byzantine Failures.

Optimal Storage under Unsynchronized Mobile Byzantine Faults

In this paper we prove lower and matching upper bounds for the number of servers required to implement a regular shared register that tolerates unsynchronized Mobile Byzantine failures. We consider the strongest model of Mobile Byzantine failures to date: agents are moved arbitrarily by an omniscient adversary from a server to another in order to deviate their computation in an unforeseen manner. When a server is infected by an Byzantine agent, it behaves arbitrarily until the adversary decides to move the agent to another server. Previous approaches considered asynchronous servers with synchronous mobile Byzantine agents (yielding impossibility results), and synchronous servers with synchronous mobile Byzantine agents (yielding optimal solutions for regular register implementation, even in the case where servers and agents periods are decoupled). We consider the remaining open case of synchronous servers with unsynchronized agents, that can move at their own pace, and change their pace during the execution of the protocol. Most of our findings relate to lower bounds, and characterizing the model parameters that make the problem solvable. It turns out that unsynchronized mobile Byzantine agent movements requires completely new proof arguments, that can be of independent interest when studying other problems in this model. Additionally, we propose a generic server-based algorithm that emulates a regular register in this model, that is tight with respect to the number of mobile Byzantine agents that can be tolerated. Our emulation spans two awareness models: servers with and without self-diagnose mechanisms. In the first case servers are aware that the mobile Byzantine agent has left and hence they can stop running the protocol until they recover a correct state while in the second case, servers are not aware of their faulty state and continue to run the protocol using an incorrect local state.

On the Bitcoin Limitations to Deliver Fairness to Users

While current Bitcoin literature mainly focuses on miner behaviors, little has been done to analyze user participation. Because Bitcoins users do not benefit from any incentive, their participation in the system is conditional upon system ability to provide a transactional service at a reasonable cost and acceptable quality. A recent observed trend on a growing number of unconfirmed transactions seems, however, to substantiate that Bitcoin is facing service degradation. The objective of this paper is to shed some light on user participation in Bitcoin against a notion of system fairness, through a utility-based approach. We first introduce fairness to quantify the satisfaction degree of participants (both users and miners) with respect to their justified expectations over time. We then characterize user strategies, deriving the necessary condition for fairness, and we show Bitcoin limitations in delivering it. The utility-based model allows to finally draw conclusions on possible improvements for fairness to promote user participation.

An Attack Graph-based On-line Multi-step Attack Detector

Modern distributed systems are characterized by complex deployment designed to ensure high availability through replication and diversity, to tolerate the presence of failures and to limit the possibility of successful compromising. However, software is not free from bugs that generate vulnerabilities that could be exploited by an attacker through multiple steps. This paper presents an attack-graph based multi-step attack detector aiming at detecting a possible on-going attack early enough to take proper countermeasures through; a Visualization interfaced with the described attack detector presents the security operator with the relevant pieces of information, allowing a better comprehension of the network status and providing assistance in managing attack situations (i.e., reactive analysis mode). We first propose an architecture and then we present the implementation of each building block. Finally, we provide an evaluation of the proposed approach aimed at highlighting the existing trade-off between accuracy of the detection and detection time.

Brief Announcement: Optimal Self-stabilizing Mobile Byzantine-Tolerant Regular Register with Bounded Timestamps

This paper investigates on the implementation of a self-stabilizing regular register emulated by n servers that is tolerant to both mobile Byzantine agents, and transient failures in a round-free synchronous model. Differently from existing Mobile Byzantine tolerant register implementation, this paper considers a more powerful adversary where (i) the message delay (i.e., \(\delta \)) and the period of mobile Byzantine agents movement (i.e., \(\varDelta \)) are completely decoupled and (ii) servers are not aware of their state i.e., they do not know if they have been corrupted or not by a mobile Byzantine agent.

Building Regular Registers with Rational Malicious Servers and Anonymous Clients

The paper addresses the problem of emulating a regular register in a synchronous distributed system where clients invoking

Approximate Agreement under Mobile Byzantine Faults Model

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Brief announcement: Approximate Agreement under Mobile Byzantine Faults Model

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Optimal Mobile Byzantine Fault Tolerant Distributed Storage: Extended Abstract

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