- A
Replace ECDH with Diffie-Hellman using 4096-bit prime modulus and use RSA-4096 signatures.
Why wrong: Larger key sizes do not offer quantum resistance and do not address authentication beyond traditional PKI.
- B
Implement ECDH with Edwards-curve Digital Signature Algorithm (EdDSA) using Curve25519 and hash-based signatures for long-term keys.
Why wrong: EdDSA with Curve25519 is not quantum-resistant; hash-based signatures are unsuitable for ephemeral key exchange.
- C
Use pre-shared keys (PSK) for both key exchange and encryption, eliminating the need for public key authentication.
Why wrong: PSK requires out-of-band secure distribution and breaks perfect forward secrecy.
- D
Use ECDH with RSA-2048 signatures for authentication and plan to migrate to CRYSTALS-KYBER when standardized.
This hybrid approach provides immediate authentication and a clear path to quantum resistance.
Quick Answer
The correct recommendation is to use ECDH with RSA-2048 signatures for authentication and plan to migrate to CRYSTALS-KYBER when standardized. This hybrid post-quantum cryptography approach immediately closes the man-in-the-middle vulnerability by authenticating the key exchange, while the forward-looking migration to CRYSTALS-KYBER ensures resistance to quantum computing attacks that would eventually break RSA-2048. On the CompTIA SecurityX CAS-004 exam, this scenario tests your understanding that perfect forward secrecy and quantum resistance are separate concerns—ECDH provides forward secrecy, but without authentication, it is insecure; RSA-2048 solves authentication now, but is not quantum-safe. A common trap is assuming larger Diffie-Hellman groups or pre-shared keys solve both issues, but they either lack quantum resistance or break forward secrecy. Remember the mnemonic: “Authenticate now, quantize later”—always pair current authentication with a planned post-quantum upgrade like CRYSTALS-KYBER.
CAS-004 Security Engineering Practice Question
This CAS-004 practice question tests your understanding of security engineering. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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.
Option A is correct because it immediately addresses the lack of authentication by using RSA-2048 signatures while also planning for quantum resistance by migrating to CRYSTALS-KYBER, a NIST-standardized post-quantum key encapsulation mechanism. This hybrid approach ensures security now and in the future. Option B is wrong because larger Diffie-Hellman parameters do not provide quantum resistance and do not solve authentication. Option C is wrong because pre-shared keys break perfect forward secrecy and are not scalable. Option D is wrong because EdDSA with Curve25519 is not quantum-resistant, and hash-based signatures are not practical for real-time key exchange.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CAS-004 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this CAS-004 question test?
Security Engineering — This question tests Security Engineering — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Use ECDH with RSA-2048 signatures for authentication and plan to migrate to CRYSTALS-KYBER when standardized. — Option A is correct because it immediately addresses the lack of authentication by using RSA-2048 signatures while also planning for quantum resistance by migrating to CRYSTALS-KYBER, a NIST-standardized post-quantum key encapsulation mechanism. This hybrid approach ensures security now and in the future. Option B is wrong because larger Diffie-Hellman parameters do not provide quantum resistance and do not solve authentication. Option C is wrong because pre-shared keys break perfect forward secrecy and are not scalable. Option D is wrong because EdDSA with Curve25519 is not quantum-resistant, and hash-based signatures are not practical for real-time key exchange.
What should I do if I get this CAS-004 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CAS-004 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 2026
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