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
82 practiced
In a long-lived connection such as a TLS session, what are post-handshake key updates? Why are they used, and what safety properties must a protocol preserve when performing post-handshake rekey (for example ordering guarantees, rollback prevention, and rate-limiting)?
HardTechnical
95 practiced
You are handed a proprietary elliptic curve (parameters provided) used for ECDH with little documentation. Enumerate and prioritize steps you would take to assess whether the curve is safe for key establishment: statistical tests for randomness of coefficients, checking twist security, group order factorization, potential endomorphisms, and known special-curve fast paths. What are red flags that would lead you to reject the curve?
HardTechnical
94 practiced
Analyze implementation strategies for constant-time scalar multiplication on elliptic curves used in ECDH. Compare fixed-window methods with blinding, the Montgomery ladder, and double-and-add-always regarding timing, simple power analysis and side-channel resistance, and resource trade-offs for constrained devices.
MediumTechnical
106 practiced
Describe a practical MAC-based key confirmation mechanism suitable for IKE/IPsec that allows both peers to confirm possession of the same keys without revealing them. Specify exact inputs to the MAC, how to include session identifiers and nonces, and how to prevent reflection and replay attacks.
MediumTechnical
154 practiced
Design a comprehensive test suite to validate DH/ECDH implementation correctness in a cryptographic library. Include unit tests for parameter validation, subgroup checks, invalid encodings, deterministic test vectors, interoperability vectors, fuzz targets, and CI integration. Specify what failure modes the tests must detect.
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