Security Design for a Chat App: Threat Model, Encryption, Accounts and Abuse

You are the product manager for a chat and messaging application, and you are asked to design its security. Explain what the product needs to protect, who it needs protection from, and which protections you would build into it. ```hint Start with the threat model List what is worth stealing or tampering with in a chat product, and who might try. Every protection you propose should answer one of those threats, and each one may take a feature away from somewhere else. ``` ### Clarifying Questions - Is this a consumer app, a workplace tool, or both? A workplace customer may need to keep and search messages, which limits how encryption can work. - Which features are in scope: one-to-one chats, large groups, file and media sharing, voice and video calls? - Must users read their history on several devices, and restore it on a new phone? - Which regulations or customer commitments apply, such as data residency or records retention? ### What a Strong Answer Covers - A threat model: the assets (message content, metadata, accounts, contacts, files) and the adversaries (network attackers, account thieves, a compromised server or insider, abusive users). - Account and device security, including takeover prevention and recovery. - Message protection in transit, at rest and end to end, and what end-to-end encryption means for groups, multiple devices, backups and search. - How spam control, abuse reporting and compliance needs work alongside encryption. - Phasing, user-experience costs, and how security is measured and incidents are handled. ### Follow-up Questions - A large business customer wants end-to-end encryption and also wants its compliance team to search every message. How do you respond? - A user loses their only phone. What can they recover, and what does that choice cost in security? - How would you let users report harassment in an encrypted chat without giving the company access to every conversation?