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I hold a Masters degree in Informatics and Telecommunications from Arts, Sciences and Technology University, Lebanon.I hold Bachelor degree in Informatics and Technology from ULF University, Lebanon.I am a PhD Student at MAPi (A joint Doctoral Programme in Computer Science of the University of Minho, the University of Aveiro and the University of Porto), I am also a researcher at HASLab/INESC TEC located in Minho University, Braga, Portugal.

I am interested in the domain of distributed systems. I am working on my thesis now, the subject is: “Extending Conflict-free Replicated Data Types Fault Models".

The thesis aims to extend the fault model of CRDTs to address Byzantine faults, rational behaviors, etc.This work aims at extending the fault model addressed in literature for CRDTs by suggestions considering Byzantine faults as it represents the strongest fault model, it exists in practice, and it has been studied very well in literature.

My supervisors are: Dr. Ali Shoker and Prof. José Bacelar Almeida.

Hosting Research Unit : High-Assurance Software Lab (HASLab)/INESC TEC & Univ. of Minho.

External Monitor : Prof. Alysson Bessani, universidade de Lisboa



  • Name

    Houssam Ahmad Yactin
  • Cluster

    Computer Science
  • Role

    External Student
  • Since

    15th March 2016


On the feasibility of byzantine agreement to secure fog/edge data management

Shoker, A; Yactine, H;

Advances in Information Security

Fog/Edge computing improves the latency and security of data by keeping storage and computation close to the data source. Nevertheless, this raises other security challenges against malicious, a.k.a, Byzantine, attacks that can exploit the isolation of nodes, or when access to distributed data is required in untrusted environments. In this work, we study the feasibility of deploying Byzantine Agreement protocols to improve the security of fog/edge systems in untrusted environments. In particular, we explore existing Byzantine Agreement protocols, heavily developed in the Blockchain area, emphasizing the Consistency, Availability, and Partition-Tolerance tradeoffs in a geo-replicated system. Our work identifies and discusses three different approaches that follow the Strong Consistency, Eventual Consistency, and Strong Eventual Consistency models. Our conclusions show that Byzantine Agreement protocols are still immature to be used by fog/edge computing in untrusted environment due to their high finality latency; however, they are promising candidates that encourage further research in this direction. © 2021, Springer Nature Switzerland AG.


Semantic Wikis Distributed on Structured Peer-to-Peer Networks

Rahhal, C;

The International Journal of E-Learning and Educational Technologies in the Digital Media



As Secure as Possible Eventual Consistency: Work in Progress

Shoker, A; Yactine, H; Baquero, C;

Proceedings of the 3rd International Workshop on Principles and Practice of Consistency for Distributed Data, PaPoC@EuroSys 2017, Belgrade, Serbia, April 23 - 26, 2017

Eventual consistency (EC) is a relaxed data consistency model that, driven by the CAP theorem, trades prompt consistency for high availability. Although, this model has shown to be promising and greatly adopted by industry, the state of the art only assumes that replicas can crash and recover. However, a Byzantine replica (i.e., arbitrary or malicious) can hamper the eventual convergence of replicas to a global consistent state, thus compromising the entire service. Classical BFT state machine replication protocols cannot solve this problem due to the blocking nature of consensus, something at odd with the availability via replica divergence in the EC model. In this work in progress paper, we introduce a new secure highly available protocol for the EC model that assumes a fraction of replicas and any client can be Byzantine. To respect the essence of EC, the protocol gives priority to high availability, and thus Byzantine detection is performed off the critical path on a consistent data offset. The paper concisely explains the protocol and discusses its feasibility. We aim at presenting a more comprehensive and empirical study in the future. © 2017 ACM.