End To End Encryption System Design Questions
Architectural design of systems providing encryption from source to destination. Key considerations: clear threat model definition, selection of encryption algorithms for different data types and threat levels, protocol design for secure communication, authentication mechanisms, integrity checking, managing forward/backward secrecy, and scalability to large user bases and data volumes. Understanding different deployment models (client-side, server-side, hybrid) and their security tradeoffs. Design considerations for systems protecting messages at rest and in transit.
HardSystem Design
78 practiced
Design an end-to-end encrypted group chat architecture for very large groups (tens of thousands of members) with asynchronous participation. Your design should minimize the amount of re-encryption and round trips on membership changes while supporting history secrecy (new members cannot read past messages) and forward secrecy. Explain how you will represent group state, how messages are encrypted to members, and how you handle join/leave operations.
MediumTechnical
80 practiced
Explain the components and state machine of the Double Ratchet algorithm (DH ratchet, symmetric-key ratchet, message keys, skipped-message keys). Describe how the algorithm copes with out-of-order and lost messages and quantify the storage and computational costs for a client maintaining n active sessions.
MediumSystem Design
100 practiced
How would you architect the server-side infrastructure for an E2EE messaging service that stores only ciphertext and minimal metadata, while supporting 100M users and 10B messages/day? Discuss components (message queues, storage tiers, indexing, notification services), partitioning/sharding strategy, and approaches to minimize latency and storage cost without introducing new cryptographic trust assumptions.
HardTechnical
92 practiced
Design an internal auditing and forensic capability for an organization operating an E2EE service that preserves user privacy yet allows abuse investigation under strict controls. Propose cryptographic constructs (e.g., sealed logs, threshold decryption, multi-party approval), governance processes, and safeguards to prevent misuse by insiders. Explain how the system preserves verifiability and tamper-evidence.
EasyTechnical
89 practiced
Define perfect forward secrecy (PFS) and explain how it protects past communications when long-term keys are compromised. Give one concrete key-exchange mechanism that provides PFS and describe the minimal steps required to obtain PFS during initial session setup.
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