Which scenario most effectively demonstrates the combined use of Diffie‑Hellman and HMAC to secure communications?

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Question restatement: Which scenario most effectively demonstrates the combined use of Diffie-Hellman and HMAC to secure communications?

Option A: 'Employing Diffie‑Hellman for encryption and using asymmetric digital signatures solely for message authentication.' This option describes using DH to encrypt data and relies only on signatures for authentication. It misuses DH for encryption (DH provides a shared secret, not direct encryption) and ignores the role of an integrity check like an HMAC, making it incomplete for achieving both confidentiality and strong authentication.

Option B: 'Relying on a fixed symmetric key for all transmissions while periodically using Diffie‑Hellman to verify message integrity.' A fixed symmetric key undermines the purpose of DH, which is to establish fresh, forward-secret session keys. Periodically verifying integrity with DH in this way does not coherently combine DH with per-message authentication, and it implies a static key elsewhere, which is insecure.

Option C: 'Using Diffie‑Hellman to establish a shared secret, then applying HMAC to each transmitted message with that key to ensure integrity and authenticity.' This is the correct approach: DH provides a secure, forward-secret shared secret, and HMAC uses that secret to provide per-message integrity and authenticity guarantees. It combines key exchange with a cryptographic MAC, aligning with best practices for secure communications.

Option D: 'Directly encrypting messages with a Diffie‑Hellman derived key without additional authentication measures.' While DH-derived keys can be used for encryption, omitting an authentication mechanism like an HMAC leaves the channel vulnerable to active attacks (e.g., MITM). This option fails to provide message integrity and authenticity, which are essential alongside confidentiality.

In summary, option C best captures the synergistic use of Diffie-Hellman for establishing a secure, forward-secret shared secret and HMAC for per-message integrity and authenticity, whereas the other options introduce gaps or misapplications of these primitives.

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Which scenario most effectively demonstrates the combined use of Diffie‑Hellman and HMAC to secure communications?

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