PhD graduated
Team : SYEL
    Sorbonne Université - LIP6
    Boîte courrier 169
    Couloir 24-25, Étage 5, Bureau 520
    4 place Jussieu
    75252 PARIS CEDEX 05

Tel: +33 1 44 27 71 23, Kahina.Khacef (at)

Supervision : Salima BENBERNOU

Co-supervision : OUZIRI Mourad

Trade-off Between Security and Scalability in Blockchain Systems

The development of Blockchain has enabled the emergence of high technology in the sensitive and active sectors by allowing the reliability of information via consensus, the immutability of records, and transaction transparency. This thesis presents the design, implementation, and evaluation of techniques to scale the blockchain.
The first part of this thesis consists of building a decentralized, secure peer-to-peer messaging protocol using a PKI-based blockchain, which can be an email, a website, or some other form of message. Managing users’ identities by the Blockchain eliminates the single point of failure of traditional PKIs. By using smart contracts to validate, store and revoke the certificate on a public blockchain. Security and scalability are considered two significant challenges in blockchains’ rapid and smooth deployment in businesses, enterprises, and organizations. The ability to scale up a blockchain lies mainly in improving the underlying technology rather than deploying new hardware. The second contribution of the thesis proposes SecuSca, an approach that makes a trade-off between security and scalability when designing blockchain-based systems. It designs an efficient replication model, which creates dynamic sharding wherein blocks are stored in various nodes. To maintain the required level of security, the proposed approach shows that blockchain replication over the Peer_to_Peer network is minimized as the blockchain’s length evolves.
Furthermore, a sharding protocol over the network is proposed to get access to the blockchain data based on historical transactions. The protocol reduces old blocks’ replication; these blocks can be discarded from specific nodes and stored by others.
The nodes willing to store the coming blocks and their data are chosen randomly.
The block header of each block is kept to achieve consensus. Next, we optimize the latest approach by choosing the entering nodes following the nodes’ capacities instead of randomly. The last part of this thesis is motivated by a security flaw in the design of the latest approach. We provide a theorem guaranteeing the security of the history maintenance mechanism against attacks. Furthermore, we prove results on the block replication at scale. We also study the appropriate parameters to select in practice and how this allows achieving 5000 transactions per second while allowing miners to participate with only 200 GB per year to store, making the protocol a great candidate to solve the blockchain trilemma by achieving decentralization on top of being scalable thanks to large blocks.

Defence : 12/09/2022 - 15h - 45 Rue des Saints-Pères 75006 Paris, Salle Turing, au 7e étage

Jury members :

Azzag Hanene MCF HDR Université Sorbonne Paris Nord [Rapporteur]
Valentina Dragos CR-HDR ONERA, [Rapporteur]
Daniela Grigori Professeur Université Paris Dauphine
Walid Gaaloul Professeur à Polytechnique de Paris
Mourad Ouziri MCF Université Paris Cité
Salima Benbernou Professeur Université Paris Cité

2019-2022 Publications