In order to ensure the interoperability of contact tracing apps within the EU and secure exchange of related data, eHealth Network published the architectural specification of the Federation Gateway Service, comprising its general functioning, interface specification, data structures, security aspects, traffic volume estimates, and storage options. Two documents -Security Architecture for contact tracing apps and Interoperability Architecture for contact tracing and warning apps state how the national backend uploads the keys of newly infected citizens ("diagnosis keys") every couple of hours and downloads the diagnosis keys from the other countries participating in this scheme. Data conversion and filtering is done in the national backends.