[$] Open-source contact tracing, part 2

Contact tracing is a way to help prevent the spread of a disease, such asCOVID-19, by identifying an infected person's contacts so that theycan be informed of the infection risk.In the first part of thisseries, we introduced open-source contact-tracing applications developed inresponse to the current pandemic, and described how they work. In thispart, we look into the details of some of them, of both centralized anddecentralized design. These application projects have all released theirsource code, but they differ in the implementation details, licenses used,and whether they accept user requests or patches. We conclude withthe controversies around the tracing applications and the responses to them.