The answer is cipher AES-256-GCM and auth SHA256. This is the best replacement pair because AES-256-GCM is an Authenticated Encryption with Associated Data (AEAD) cipher that inherently combines encryption and integrity verification, making a separate auth directive redundant yet fully compatible with modern TLS 1.3 requirements. On the CompTIA SecurityX CAS-004 exam, this scenario tests your understanding of cryptographic modernization—specifically that outdated ciphers like Blowfish, DES, or 3DES and deprecated hashes like MD5 must be replaced with strong, AEAD-based suites. A common trap is assuming that adding HMAC to an already-secure SHA256 auth improves security, but the real best practice is to adopt a cipher that handles both tasks, eliminating unnecessary overhead. Remember the mnemonic: “AEAD does it all—no extra auth needed.”
CAS-004 Security Engineering Practice Question
This CAS-004 practice question tests your understanding of security engineering. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
Exhibit:
```
[openvpn]
port 1194
proto udp
dev tun
ca ca.crt
cert server.crt
key server.key
dh dh2048.pem
tls-server
tls-version-min 1.2
cipher AES-256-CBC
auth SHA256
```
An OpenVPN configuration file is shown. A security auditor recommends replacing the cipher and auth directives. Which of the following is the BEST replacement pair from a security engineering perspective?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Refer to the exhibit.
Exhibit:
```
[openvpn]
port 1194
proto udp
dev tun
ca ca.crt
cert server.crt
key server.key
dh dh2048.pem
tls-server
tls-version-min 1.2
cipher AES-256-CBC
auth SHA256
```
A
cipher AES-256-GCM and auth SHA256
AES-256-GCM is an AEAD cipher that includes authentication, so the auth directive becomes unnecessary; however, OpenVPN allows both. This is a secure modern combination.
B
cipher AES-128-GCM and auth SHA384
Why wrong: While GCM is good, AES-128 is less secure than AES-256; also auth SHA384 is stronger but unnecessary if GCM is used. The recommendation prefers stronger encryption.
C
cipher 3DES-168 and auth MD5
Why wrong: 3DES is obsolete and vulnerable to meet-in-the-middle attacks; MD5 is cryptographically broken.
D
cipher Blowfish-128 and auth SHA1
Why wrong: Blowfish is relatively old and less efficient than AES; SHA1 is deprecated for security.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
cipher AES-256-GCM and auth SHA256
Option C is correct because AES-256-GCM is an AEAD cipher that provides both encryption and authentication, and TLS 1.3 uses AEAD. Option A is wrong because the configuration already uses SHA256 for auth; adding HMAC is redundant but not best. Option B is wrong because Blowfish is outdated and DES is weak. Option D is wrong because 3DES is weak and MD5 is deprecated.
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.
✓
cipher AES-256-GCM and auth SHA256
Why this is correct
AES-256-GCM is an AEAD cipher that includes authentication, so the auth directive becomes unnecessary; however, OpenVPN allows both. This is a secure modern combination.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
cipher AES-128-GCM and auth SHA384
Why it's wrong here
While GCM is good, AES-128 is less secure than AES-256; also auth SHA384 is stronger but unnecessary if GCM is used. The recommendation prefers stronger encryption.
✗
cipher 3DES-168 and auth MD5
Why it's wrong here
3DES is obsolete and vulnerable to meet-in-the-middle attacks; MD5 is cryptographically broken.
✗
cipher Blowfish-128 and auth SHA1
Why it's wrong here
Blowfish is relatively old and less efficient than AES; SHA1 is deprecated for security.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 CAS-004 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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: cipher AES-256-GCM and auth SHA256 — Option C is correct because AES-256-GCM is an AEAD cipher that provides both encryption and authentication, and TLS 1.3 uses AEAD. Option A is wrong because the configuration already uses SHA256 for auth; adding HMAC is redundant but not best. Option B is wrong because Blowfish is outdated and DES is weak. Option D is wrong because 3DES is weak and MD5 is deprecated.
What should I do if I get this CAS-004 question wrong?
Identify which CAS-004 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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