Google Google This document specifies how users can privately discover each other's Service Specific Identifiers (SSIs) when using end-to-end encrypted messaging services across multiple providers. Users can retrieve SSIs without revealing their social graphs to service providers they are not delivering messages through, using their phone numbers, email, user IDs, or other Service Independent Identifiers (SIIs). Our specification can be based on private information retrieval or associative private sets membership schemes, both of which provide reasonable tradeoffs between privacy and cost.