Contact tracing applications have flooded the marketplace, as governments worldwide have been working to release apps for their citizens. These apps use a variety of protocols to perform contact tracing, resulting in widely differing security and privacy assurances. Governments and users have been left without a standard metric to weigh these protocols and compare their assurances to know which are more private and secure. Although there are many ways to approach a quantitative metric for privacy and security, one natural way is to draw on the methodology used by the well-known common vulnerability scoring system (CVSS). For privacy, we applied consensus principles for contract tracing as a basis for comparing their relative privacy practices. For security, we performed attack modeling to develop a rubric to compare the security of respective apps. Our analysis shows that centralized Bluetooth with added location functionality has low privacy and security, while non-streaming GPS scored high in security and medium in privacy. Based on our methodology, only two apps were given a high ranking of privacy: Canada’s Covid Alert and Germany’s Corona Warn-App. They both used the Google/Apple Notification Framework as the basis for their design. To achieve comparable privacy, we recommend that future projects follow their examples in the following ways: minimizing the amount of data they collect and holding it for the shortest possible length of time; only having features necessary for the app’s main function; and releasing design details so that users can make informed decisions.
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