Key Establishment and Agreement Questions
Deep understanding of key exchange mechanisms including Diffie-Hellman, ECDH, and modern constructions using KDFs (HKDF). Understanding of parameter negotiation, protection against downgrade attacks, forward secrecy properties. Knowledge of key confirmation mechanisms and post-handshake key updates. Awareness of post-quantum key exchange candidates and transition strategies.
EasyTechnical
107 practiced
Compare ephemeral-ephemeral, ephemeral-static, and static-static key agreement patterns. For each pattern, state typical security properties (authentication and forward secrecy) and practical trade-offs for server-client deployments, including computational cost and certificate usage.
HardSystem Design
80 practiced
Design a scalable group key agreement protocol for large, dynamic groups (thousands of members) that provides contributory key agreement and forward secrecy when members join or leave. Compare tree-based approaches (for example TreeKEM / MLS) with broadcast KEM approaches, discussing message and computation complexity for joins and leaves.
EasyTechnical
73 practiced
What is key confirmation in a key-exchange protocol? Give two mechanisms for mutual key confirmation (for example, a MAC over the transcript using the derived key, and an explicit signature over the key material) and explain how key confirmation prevents certain active and reflection attacks.
MediumSystem Design
90 practiced
Design a minimal two-message authenticated key exchange using ephemeral Diffie-Hellman for both parties plus long-term signature-based authentication for the server and client. Specify message formats, what is signed, when the KDF is applied, and explain how the design provides both authentication and perfect forward secrecy. Discuss replay protections.
EasyTechnical
97 practiced
Define forward secrecy and perfect forward secrecy in the context of key agreement protocols. Explain how use of ephemeral keys and regular key rotation achieve these properties. Give two concrete protocol examples that provide forward secrecy by default.
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