The answer is ESP in tunnel mode with null encryption. This is the correct choice because Encapsulating Security Payload (ESP) with null encryption, defined in RFC 2410 as ESP_NULL, provides strong authentication and integrity via an HMAC such as HMAC-SHA256 while deliberately omitting any data encryption, perfectly matching the requirement for a VPN protocol for authentication and integrity without confidentiality. On the CompTIA SecurityX CAS-004 exam, this scenario tests your understanding that IPsec’s ESP can operate in a “sign-only” mode, a common trap where candidates mistakenly choose AH (Authentication Header) instead—but AH cannot traverse NAT devices and lacks the flexibility of ESP’s null encryption. Remember the memory tip: “ESP_NULL signs the payload, AH signs the whole packet but breaks NAT.”
CAS-004 Security Engineering Practice Question
This CAS-004 practice question tests your understanding of security engineering. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. 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 security architect is designing a VPN solution for remote employees. The company requires strong authentication and integrity protection but is less concerned about confidentiality for non-sensitive traffic. Which protocol is most appropriate?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ESP in tunnel mode with null encryption
Option C is correct because ESP in tunnel mode with null encryption provides authentication and integrity via HMAC (e.g., HMAC-SHA256) while omitting encryption (ESP_NULL, RFC 2410). This satisfies the requirement for strong authentication and integrity without confidentiality for non-sensitive traffic, as the payload is authenticated but not encrypted.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
L2TP/IPsec with ESP
Why it's wrong here
L2TP/IPsec typically encrypts all traffic; not suitable if confidentiality is not needed.
✗
SSL/TLS VPN
Why it's wrong here
SSL/TLS VPNs provide encryption; overkill and not the best fit for this requirement.
✓
ESP in tunnel mode with null encryption
Why this is correct
ESP can be configured with null encryption to provide integrity and authentication without confidentiality.
Related concept
Read the scenario before looking for a memorised answer.
✗
AH in transport mode
Why it's wrong here
AH provides integrity and authentication but not encryption; cannot be used with null encryption.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume ESP always requires encryption, but ESP_NULL (RFC 2410) explicitly allows authentication-only mode, making it ideal when confidentiality is not needed.
Detailed technical explanation
How to think about this question
ESP in tunnel mode encapsulates the entire original IP packet as a payload, adding a new IP header and ESP trailer. With null encryption (ESP_NULL), the payload is sent in plaintext but is still authenticated via an Integrity Check Value (ICV), allowing the receiver to verify data origin and integrity without decryption. This is commonly used in scenarios like DMZ-to-DMZ traffic where confidentiality is not required but integrity is critical, as defined in RFC 4303.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the CAS-004 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Security Engineering — This question tests Security Engineering — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ESP in tunnel mode with null encryption — Option C is correct because ESP in tunnel mode with null encryption provides authentication and integrity via HMAC (e.g., HMAC-SHA256) while omitting encryption (ESP_NULL, RFC 2410). This satisfies the requirement for strong authentication and integrity without confidentiality for non-sensitive traffic, as the payload is authenticated but not encrypted.
What should I do if I get this CAS-004 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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