AI & Technical question
Design the security for a chat/messaging application.
Practice this question out loud. An AI interviewer asks it, follows up like a real interviewer would, and scores your answer. Type or speak.
Start a mock interview on this question · Mock interview from a job description
What this question tests
Tests technical system design for security, covering encryption, authentication and abuse prevention, and the trade-off between security and usability.
How to approach it
- Clarify the threat model: end-to-end encrypted consumer chat versus enterprise chat with compliance needs, since the design differs.
- Define core message confidentiality using end-to-end encryption, such as a Double Ratchet style protocol.
- Add authentication and device management: key verification on new device sign-in and safety-number checks between users.
- Address abuse separately from encryption: spam and phishing detection, report or block flows, and metadata-based rate limiting since content is encrypted.
- Handle the account recovery trade-off so encrypted backups don't create a de facto backdoor.
- Define success as zero plaintext exposure and a low false-positive rate on abuse detection.
What a strong answer includes
- Separates message confidentiality from platform safety, which are different problems needing different tools.
- Explains the real trade-off: end-to-end encryption limits server-side moderation, so abuse detection leans on metadata and user reports.
- Uses an illustrative number as an assumption: assume 0.1% of messages get flagged via reports.
- Names key management as the hardest part, since losing a device shouldn't mean losing history, but backups must stay encrypted client-side.
Common mistakes
- Proposing encryption without addressing key management and device changes.
- Ignoring abuse and spam as a distinct problem from confidentiality.
- Assuming server-side content scanning is compatible with true end-to-end encryption.
Likely follow-up questions
- How do you handle a report on a message you can't read?
- How do you support multi-device without weakening security?
- What's your incident response if a protocol vulnerability is found?
More ai & technical questions
- Create an API design for third-party integration for payments.Microsoft · AI & Technical · Hard
- How will you design a web service that enables other developers to create online chat applications?Microsoft · AI & Technical · Hard
- How will you design a mobile version of MS Word and ensure real time collaboration within this app?Microsoft · AI & Technical · Hard
- Write an algorithm for a robot that has to get from point A to point B in a room.Microsoft · AI & Technical · Medium
- Design a system that can detect fraudulent use of Microsoft Word.Microsoft · AI & Technical · Hard
- Explain the architecture of your current or past product in detail.Flexport · AI & Technical · Easy
More questions from these companies
Learn the skill behind it
Chapters of the AI PM course that teach what this question tests.
- Chapter 1: Foundations: the model and the decisions it forces on you
- Chapter 8: Evals: define good and make the number defensible
- Chapter 6: Agents and agentic architecture