End-to-end encrypted messaging
Design the backend for an end-to-end encrypted messaging service with 200M users supporting 1:1 and group chats, multi-device, and offline delivery — where the server itself must never be able to read message content. It must deliver messages with p99 under 200ms when recipients are online, queue for offline devices, and provide forward secrecy so that compromising a device's current key doesn't decrypt past messages. Cover key distribution, the encryption/trust model, group messaging, and multi-device.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify6:00 left
Estimate6:00 planned
Design18:30 planned
Deep dive14:30 planned
Failure10:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.