This document describes a secure mechanism to join sets of heterogeneous ids from multiple data providers (e.g. broadcasters, publishers, data enrichment providers) and create a set of encrypted common identifiers, which we refer to as SUMIDs. These identifiers can be used for computing multi-party reach, frequency, sales lift, MTA, and other ads metrics. We also introduce the concept of “match rules”, which dictate when two heterogeneous IDs should be assigned the same SUMID within the secure mechanism. We avoid prescribing specific match rules as these could vary depending upon a number of considerations, such as the specific media market or whether the SUMID is intended to represent a household or an individual. Optimal choice of match rules are also an open area of research.