Group Key Management for Large Scale Collaboration

Le descriptif de l’offre ci-dessous est en Anglais_

**Type de contrat**: CDD

**Niveau de diplôme exigé**: Thèse ou équivalent

**Fonction**: Ingénieur scientifique contractuel

**Contexte et atouts du poste**:
About Hivenet:
Hivenet intends to play the role of a next generation cloud provider in the context of Web 3.0. Hivenet aims to exploit the unused capacity of computers to offer the general public a greener and more sovereign alternative to the existing clouds where the true power lies in the hands of the users. It relies both on distributed peer-to-peer networks and on the encryption of end-to-end data.

About Inria Center of the University of Lorraine:
The Inria Center of the University of Lorraine is one of Inria's nine centers and has twenty project teams, located in Nancy, Strasbourg and Saarbrücken. Its activities occupy over 400 people, scientists and research and innovation support staff, including 45 different nationalities. The Inria Center is a major and recognized player in the field of digital sciences. It is at the heart of a rich R&D and innovation ecosystem: highly innovative PMEs, large industrial groups, competitiveness clusters, research and higher education players, laboratories of excellence, technological research institutes, etc.

About Inria Center of the University of Rennes:
The Inria Center of the University of Rennes is one of Inria's eight centers and has more than thirty research teams. The Inria Center is a major and recognized player in the field of digital sciences. It is at the heart of a rich R&D and innovation ecosystem: highly innovative PMEs, large industrial groups, competitiveness clusters, research and higher education players, laboratories of excellence, technological research institutes, etc.

**Mission confiée**:
In a large scale collaboration where several users join and leave very often the group, the generation and sharing of group keys is critical. Suppose that the group decides to eliminate a user from the group. This user should not have access to the document anymore and a new group key has to be generated and shared among the remaining group members which requires several rounds of communication among users. This process raises performance issues and leads to interruptions of participants’ work.

Solutions for n-party group communication remained either inefficient or less secure until the standardization of the MLS Protocol (Messaging Layer Security). MLS relies on a Group Key Agreement Protocol that allows members of a group to derive a group key which serves as a basis to secure group communications. It is scalable in terms of the number of operations modifying the group state (i.e. adding/removing members and performing periodic key renewals preventing compromised communication). MLS assumes that all messages modifying the group state are delivered to all members in the same order. If two group members concurrently modify the group state this might result in a group state inconsistency that cannot be resolved. MLS requires that one of the requests is rejected and retried. This total order is enforced by routing all messages through a centralized, semi-trusted Delivery Service (DS) component. The centralization of this component makes it an ideal target for attackers who wish to disrupt communication. Notably, with the help of a compromised DS, an attacker can prevent group members from refreshing their keys and resolving the compromise.

In order to overcome these limitations, we proposed an initial design of a fully distributed DS [1]. It combines two distributed communication mechanisms adapted to the need of the messages exchanged by the protocol. We used a Probabilistic Reliable Broadcast mechanism to reliably deliver messages allowing users to propose changes to the group (i.e. Proposal messages) and a Cascade Consensus Protocol to deliver the messages that actually modify the group (i.e. Commit messages) and thus require an agreement between members.

The recruited engineer will refine, implement and evaluate the proposed solution.

References:
[1] Ludovic Paillat, Claudia-Lavinia Ignat, Davide Frey, Mathieu Turuani, Amine Ismail (2024) Discreet: distributed delivery service with context-aware cooperation. Annals of Telecommunications 2024, pp.1-23. DOI : 10.1007/s12243-024-01053-1

**Principales activités**:

- Study of group key agreement protocols: TreeKEM [2], DCGKA [3]
- Study of MLS protocol
- Study of Discreet (Distributed delivery service with context-aware cooperation) [1]
- Implement Discreet
- Design an experimental protocol for the evaluation of Discreet
- Analyse the evaluation results

**Compétences**:
PhD in Computer science / Applied mathematics with an experience in computer networks.
Theoretical expertise: distributed systems, P2P networks

Good collaborative and networking skills, excellent written and oral communication in English
Good programming skills
Strong analytical skills

**Avantages**:

- Subsidized meals
- Par