Blockchain technology has proven to be a promising solution for decentralized systems in various industries. At the core of a blockchain system is the peer-to-peer (P2P) overlay, which facilitates communication be- tween parties in the blockchain system. Recently, there is increasing evidence that this P2P overlay plays a major role in limited transaction throughput, limited scalability, and security threats in a blockchain system. In this work, it is explained that this bottleneck is caused by the fact that currently deployed P2P overlays cannot fully capture the complexity of a blockchain system. We propose a novel approach to address these issues by introducing a novel overlay architecture. Our approach, Sovereign Domains P2P Overlays, allows for nodes in a single network to belong to multiple heterogeneous groups (called domains), each with their own set of protocols tailored to the characteristics and needs of the nodes in that domain. To demonstrate the effectiveness of the proposed overlay architecture, two novel node discovery protocols, FedKad and SovKad, were designed and implemented. The simulations conducted in a custom-built simulator show that SovKad outperforms node discovery in a Structured P2P Overlay (Kademlia) and node discovery in a Federated P2P Overlay (FedKad), providing evidence of the potential of the Sovereign Domains P2P Overlay architecture in blockchain systems.

Fraudulent behavior within online marketplaces is a prominent but unsolved problem. Most marketplace operators try to mitigate this behavior by serving as the central authority. This approach requires user data collection and is not privacy-friendly. In an attempt to build a foundation for solving the fraud concerns and privacy issues, this paper elaborates on the design and implementation of a simple marketplace system using peer-to-peer (P2P) technology in combination with a Self-Sovereign Identity (SSI) solution. The P2P network ensures no single points of control, reduces risks of a big data breach and simply costs less to operate. The SSI solution makes sure that users cannot create multiple accounts to whitewash their dishonest behavior. Ensuring every user has only one identity makes the platform Sybil-resistant. In contrast to other identity verification systems used in marketplaces, such as Facebook Login, SSI aims to put the user in control and to not collect personal data. Users know what data are asked and give explicit consent for each request. This user-centric approach makes them privacy-friendly. Reaching Sybil resistance without having a central authority in a marketplace has not been done before. In the future a reputation system can be built on top of the Sybil-resistant P2P system, ensuring users’ behavior can not be whitewashed. Several methods are used during the design and the implementation process. They include the Scrum framework, MoSCoW prioritization and Class-Responsibility-Collaboration cards. Git was used for version control while code quality was kept high through a custom CI setup. Additionaly, every merge request required at least two approvals to ensure thorough code review. This resulted in an application that is both Sybil-resistant and privacy-friendly