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CAS-004 Practice Question: A defense contractor is developing a new secure…
A defense contractor is developing a new secure messaging application for classified communications. The application must ensure end-to-end encryption, perfect forward secrecy, and resistance to quantum computing attacks. The development team proposes using ECDH for key exchange and AES-256-GCM for message encryption. The security architect reviews the design and identifies a weakness: the current key exchange does not authenticate the public keys, making it vulnerable to man-in-the-middle attacks. The team suggests adding digital signatures using RSA-2048. However, the architect is concerned about quantum resistance. What should the architect recommend?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use ECDH with RSA-2048 signatures for authentication and plan to migrate to CRYSTALS-KYBER when standardized.
It provides a practical hybrid approach: using ECDH for key exchange (ensuring perfect forward secrecy) with RSA-2048 signatures for authentication (preventing man-in-the-middle attacks), while planning to migrate to the NIST-standardized post-quantum algorithm CRYSTALS-KYBER when available. This balances current security needs with future quantum resistance. Option A is wrong because increasing Diffie-Hellman parameters does not provide quantum resistance, and RSA-4096 is still vulnerable to quantum attacks. Option B is wrong because EdDSA with Curve25519 is not quantum-resistant, and hash-based signatures are unsuitable for real-time key exchange. Option C is wrong because pre-shared keys break perfect forward secrecy and are not scalable.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Replace ECDH with Diffie-Hellman using 4096-bit prime modulus and use RSA-4096 signatures.
Why it's wrong here
Larger key sizes do not offer quantum resistance and do not address authentication beyond traditional PKI.
- ✗
Implement ECDH with Edwards-curve Digital Signature Algorithm (EdDSA) using Curve25519 and hash-based signatures for long-term keys.
Why it's wrong here
EdDSA with Curve25519 is not quantum-resistant; hash-based signatures are unsuitable for ephemeral key exchange.
- ✗
Use pre-shared keys (PSK) for both key exchange and encryption, eliminating the need for public key authentication.
Why it's wrong here
PSK requires out-of-band secure distribution and breaks perfect forward secrecy.
- ✓
Use ECDH with RSA-2048 signatures for authentication and plan to migrate to CRYSTALS-KYBER when standardized.
Why this is correct
This hybrid approach provides immediate authentication and a clear path to quantum resistance.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CAS-005 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CAS-005 exam.