So how will Denmark's Covid-19 "passport" work? At least four ready-made solutions exist based broadly on two types of technology. One relies on remote cloud servers where information is stored in bulk. The other uses blockchain, a more complicated system that could be better at protecting privacy. Since personal medical data is so sensitive, it's a tricky decision. That's why many European nations covered by stringent EU privacy laws appear desperate for someone else to go first. Digital toolbox Ida Marie Odgaard/Ritzau Scanpix/AFP/Getty Images The high level of investment in developing Covid passport systems indicates high private sector optimism that they will become a common way to open borders.