Passkey authentication system
Design a passkey (WebAuthn/FIDO2) authentication system to replace passwords for a 100M-user consumer product, while keeping a graceful fallback for users who lose their device or are on an unsupported platform. Requirements: phishing-resistant login, credentials sync across a user's devices, and account recovery that doesn't reintroduce a phishable shared secret. Walk through registration and login, where the public-key credentials live and how they're looked up at login, the cross-device/sync story, and the hardest problem — account recovery.
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.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 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.