This was brutal. Apple system design interviews go deep on security and privacy.

The question was about designing E2E encryption for a messaging system like iMessage. I started with key exchange using Signal protocol (Double Ratchet), which the interviewer seemed to expect.

Where I struggled was the multi-device sync aspect. How do you handle key distribution when a user has an iPhone, iPad, and Mac all needing to decrypt the same message? I talked about device keys and pre-key bundles but didn't articulate the key transparency server well.

The follow up about group messaging encryption really tripped me up. Handling member additions and removals in encrypted groups requires tree-based key management and I hadn't studied that enough.

Also got asked about how to handle message delivery receipts without leaking metadata. Mentioned anonymous credentials but couldn't go deep enough.

Got rejected. Fair enough, I needed to study the crypto protocols more carefully. The interviewer was professional and the feedback was constructive.