System designMediumsd-g625

Subject Passwordless authLevel Mid–Senior~40 minCommon in Security interviewsIndustries Technology

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.