Design a Chat Application

Design a chat application that supports one-on-one and group messaging with user registration, real-time message delivery, and offline support. Problem Statement Design a chat application that supports one-on-one and group messaging with user registration, real-time message delivery, and offline support. This was an Apple phone screen for a Software Engineer, conducted by a security engineer who emphasized user registration (never store raw passwords), multi-region architecture, failure scenarios, and scaling.