This document specifies back-end mechanisms for Pandemic contact Tracing Systems, including architecture, information flow, protocols for sharing proximity data of contacts, data protection, user privacy, information security and the requisite APIs (Application Programming Interfaces). Sufficient technical detail will be included to also facilitate means of interoperability in a later Work Item. Each system is characterized (e.g. in a table) by its degree of compatibility with the E4P requirements work item. The initial version of this document may be revised several times as additional methods are added, e.g. including methods which are gaining consensus in non-European countries.