One of the responses to the COVID-19 pandemic consists of identifying contacts of infected people so they can be informed about the risk; that will allow them to search for medical care, if needed. This is laborious work if it is done manually, so a number of applications have been developed to help with contact tracing. But they are causing debates about their effectiveness and privacy impacts. Many of the applications were released under open-source licenses. Here, we look at the principles of these applications and the software frameworks used to build them; part two will look into some applications in more detail, along with the controversies (especially related to privacy) around these tools.