Passwordless authentication
Design a passwordless authentication system using passkeys (WebAuthn/FIDO2) for a consumer service with 60M users, replacing passwords to eliminate credential-stuffing and phishing. It must support multi-device sync of passkeys, a phishing-resistant login at 100k logins/sec, and — the hard part — a secure account-recovery path for users who lose all their devices, without reopening a phishing hole. Cover the credential model, the server side of registration/authentication, multi-device, and 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.