Nadeem Ahmed, Regio A. Michelin, Wanli Xue, Sushmita Ruj, Robert A. Malaney, Salil S. Kanhere, Aruna Seneviratne, Wen Hu, Helge Janicke, Sanjay K. Jha: A Survey of COVID-19 Contact Tracing Apps. IEEE Access 8: 134577-134601 (2020)