arXiv subjects

Rida Bazzi

Publications and source records attributed to Rida Bazzi.

5 recordsLinked to original sources

arXiv2025

Synchronization in Anonymous Networks Under Arbitrary Dynamics

We present the $\delta$-Synchronizer, which works in non-synchronous dynamic networks under minimal assumptions. Our model allows for arbitrary topological changes without any guarantee of eventual global or partial stabilization and assumes that nodes are anonymous. This deterministic synchronizer is the first that enables nodes to simulate a dynamic network synchronous algorithm for executions in a semi-synchronous dynamic environment under a weakly-fair node activation scheduler, despite the absence of a global clock, node ids, persistent connectivity or any assumptions about the edge dynamics (in both the synchronous and semi-synchronous environments). We make the following contributions: (1) we extend the definition of synchronizers to networks with arbitrary edge dynamics; (2) we present the first synchronizer from the semi-synchronous to the synchronous model in such networks; and (3) we present non-trivial applications of the proposed synchronizer to existing algorithms. We assume an extension of the Pull communication model by adding a single 1-bit multi-writer atomic register at each edge-port of a node. We show that this extension is needed and that synchronization in our setting is not possible without it. The $\delta$-Synchronizer operates with a multiplicative memory overhead at the nodes that is asymptotically logarithmic on the runtime of the underlying synchronous algorithm being simulated-in particular, it is logarithmic for polynomial-time synchronous algorithms.

cs.DC↗

arXiv2024

Secret Quorums: Protecting Byzantine Protocols Against Adaptive Adversaries

Modern committee-based payment protocols improve scalability by delegating critical operations to small subsets of participants, such as validator committees in blockchains or shard committees in distributed systems with parallel execution. This design, however, makes these protocols particularly vulnerable to adaptive adversaries: once a small set of participants is identified, it can be selectively targeted for corruption, bribery, or denial-of-service attacks. In this paper, we propose Secret Quorums, a novel abstraction that enables any committee-based protocol, including payment systems, to rely on small quorums while remaining resilient to adaptive adversaries. Validators composing a Secret Quorum remain anonymous throughout the validation: as in cryptographic sortition approaches, their selection is secret, but unlike classical approaches, the resulting quorum proof does not reveal which validators were selected. We show how to implement Secret Quorums using ring verifiable random functions, without adding communication steps compared to standard quorum-based protocols. We also demonstrate the relevance of Secret Quorums through StealthDust, a new protocol that applies Secret Quorums to the fractional spending payment problem in order to reduce latency and improve settlement message complexity with respect to the original protocol.

cs.CR↗

arXiv2024

Fractional Payment Transactions: Executing Payment Transactions in Parallel with Less than f+1 Validations

We consider the problem of supporting payment transactions in an asynchronous system in which up to $f$ validators are subject to Byzantine failures under the control of an adaptive adversary. It was shown that, in the case of a single owner, this problem can be solved without consensus by using byzantine quorum systems (requiring a quorum of $2f+1$ validations per transaction). Nonetheless, the process of validating transactions remains sequential. For example, if one has a balance of ten coins and intends to make separate payments of two coins each to two distinct recipients, both transactions must undergo processing by a common correct validator. On the other hand, these two transactions are non-conflicting as they do not lead to double spending, allowing in principle for parallel validation. In this paper, we show that it is possible to validate payment transactions in parallel with less than $f$ validations per transaction in an asynchronous system, provided that each transaction spends only a small fraction of a balance. Our solution relies on a novel class of probabilistic quorum systems that we introduce in this paper, termed \textit{$(k_1,k_2)$-quorum systems}. In the absence of an adaptive adversary, \textit{$(k_1,k_2)$-quorum systems} can be used to enable concurrent and asynchronous validation of up to $k_1$ transactions while preventing validation of more than $k_2$ transactions. Employing a $(k_1, k_2)$-quorum system, we introduce protocols enabling a payer to validate multiple \textit{fractional spending} transactions in parallel with less than $f+1$ validations per transaction. Subsequently, the payer reclaims any remaining funds through a fully validated transaction, referred to as a \textit{settlement} transaction.

cs.DC↗

arXiv2019

Clairvoyant State Machine Replication

We propose a new protocol for the generalized consensus problem in asynchronous systems subject to Byzantine server failures. The protocol solves the consensus problem in a setting in which information about conflict between transactions is available (such information can be in the form of transaction read and write sets). Unlike most prior proposals (for generalized or classical consensus), which use a leader to order transactions, this protocol is leaderless, and relies on non-skipping timestamps for transaction ordering. Being leaderless, the protocol does not need to pause for leader elections. The use of non-skipping timestamps permits servers to commit transactions as soon as they know that no conflicting transaction can be ordered earlier. For n servers of which f may be faulty, this protocol requires n > 4f.

cs.DC↗

arXiv2016

Leader Election and Shape Formation with Self-Organizing Programmable Matter

We consider programmable matter consisting of simple computational elements, called particles, that can establish and release bonds and can actively move in a self-organized way, and we investigate the feasibility of solving fundamental problems relevant for programmable matter. As a suitable model for such self-organizing particle systems, we will use a generalization of the geometric amoebot model first proposed in SPAA 2014. Based on the geometric model, we present efficient local-control algorithms for leader election and line formation requiring only particles with constant size memory, and we also discuss the limitations of solving these problems within the general amoebot model.

cs.ET↗