  This report summarizes the requirements and proposes a high-level solution
for interoperability across recently proposed COVID-19 exposure notification
efforts. Our focus is on interoperability across exposure notification (EN)
applications which are based on the decentralized Bluetooth Low Energy (BLE)
protocol driven by Google/Apple Exposure Notifications API (including DP3T and
similar protocols). We distinguish different interoperability use cases, such
as worldwide public EN interoperability, as well as interoperability in the
enterprise EN systems. This report also proposes an API and a backend
implementation architecture for EN interoperability. Finally, we propose using
a permissioned blockchain-based solution for managing EN backend certificates
and configurations (without storing any users' data on the blockchain) for
helping address EN interoperability challenges across different vendors.
