This PCNSA practice question tests your understanding of decryption and monitoring. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
```
admin@PA-500> show decryption statistics
Decryption Statistics:
Total Sessions Decrypted: 1500
Total Sessions Failed: 50
Failed Reasons:
handshake_failure: 30
certificate_unknown: 15
decryption_error: 5
```
An administrator runs the command and sees the above output. What is the most likely cause of the large number of handshake failures?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The server is using an unsupported cipher suite.
The output shows a large number of handshake failures, which typically occur during the SSL/TLS handshake when the client and server cannot agree on a common cipher suite. If the server only supports weak or outdated ciphers that the firewall's decryption policy does not allow, or if the firewall's SSL forward proxy engine does not support the server's chosen cipher, the handshake will fail. This is the most likely cause because cipher suite mismatch is a common source of handshake failures in decryption environments.
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.
✗
The firewall's CRL server is unreachable.
Why it's wrong here
Unreachable CRL would cause certificate_unknown, not handshake_failure.
✓
The server is using an unsupported cipher suite.
Why this is correct
Cipher mismatches commonly cause handshake failures.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The decryption policy is not matching the traffic.
Why it's wrong here
If the traffic doesn't match, no decryption attempt occurs.
✗
The firewall's certificate is not trusted by clients.
Why it's wrong here
Client trust issues cause certificate_unknown, not handshake_failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse handshake failures with certificate validation issues, but handshake failures specifically indicate a failure in the initial negotiation phase (e.g., cipher mismatch or protocol version incompatibility), not a trust or CRL problem.
Detailed technical explanation
How to think about this question
During an SSL/TLS handshake, the ClientHello and ServerHello messages negotiate the cipher suite; if the firewall's SSL forward proxy engine cannot support the server's selected cipher (e.g., due to policy restrictions or lack of support for that cipher in the proxy), the handshake will fail with an error such as 'handshake failure' (alert 40). In PAN-OS, the decryption profile controls allowed cipher suites, and if the server's cipher is not in the allowed list, the firewall will abort the handshake. This is distinct from certificate-related failures, which occur later in the handshake (e.g., during CertificateVerify or Finished messages).
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 PCNSA 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.
Decryption and Monitoring — This question tests Decryption and Monitoring — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The server is using an unsupported cipher suite. — The output shows a large number of handshake failures, which typically occur during the SSL/TLS handshake when the client and server cannot agree on a common cipher suite. If the server only supports weak or outdated ciphers that the firewall's decryption policy does not allow, or if the firewall's SSL forward proxy engine does not support the server's chosen cipher, the handshake will fail. This is the most likely cause because cipher suite mismatch is a common source of handshake failures in decryption environments.
What should I do if I get this PCNSA question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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