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.
MediumSystem Design
88 practiced
Design a group messaging key-management approach for groups of up to 1,000 members. Compare at least three designs: pairwise-sender-keys, centralized server re-encryption, and tree-based KEM (e.g., MLS-like). For each approach provide: join/leave cost, ciphertext size per message, history-secrecy characteristics, and an estimate of client and server work on membership changes.
MediumSystem Design
75 practiced
Design an end-to-end encrypted one-to-one messaging flow for an asynchronous system that must support offline message delivery and forward secrecy. Provide a high-level sequence diagram for (a) account/device registration, (b) initial session establishment between two devices, and (c) subsequent message sending including how skipped or out-of-order messages are handled. Specify which cryptographic primitives you would choose (KEM/AEAD/HKDF/etc.) and why.
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.
EasyTechnical
77 practiced
Describe the primary authentication options for E2EE systems: key fingerprints, certificate-based PKI, trust-on-first-use (TOFU), and cross-signing. For each option explain where trust is anchored, the user experience implications, and a typical attack that the mechanism either prevents or fails to prevent.
EasyTechnical
76 practiced
Describe the security and operational tradeoffs between encrypting data 'at rest' on the server and encrypting on the client before upload (client-side encryption). In your answer, list three server-side features that become harder to implement with strict client-side E2EE and propose a mitigation for each feature.
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