Driven by use case scenarios, WP5 aims to evaluate the results obtained in WP2 (Policy and Transparency Framework), WP3 (Big Data Policy Engine), and WP4 (User Interaction) under real world conditions. More specifically, the goal of T5.3 Public challenges is to expose the developed system and its components to public hacking challenges, which would, together with the internal backend scalability and robustness testing performed in WP3 (D3.3 and D3.5) and frontend testing in WP4 (D4.2 and D4.4), provide the necessary feedback for further development of the SPECIAL platform. In D5.2 Public Challenge Report V1, we explained that in order to guarantee the robustness of our architecture, such challenges must focus on ensuring that both the individual components and the infrastructure as a whole are capable of sharing data only with authorized parties, while guaranteeing that policies and regulations are being adhered to. We also argued that this means involving other aspects of the system, be it technical or legal, highlighting any limitations. This is worth repeating as the outcome of the three rounds of challenges has made us reconsider and adapt our strategy, as we will explain further below.