PCNSA Decryption and Monitoring Practice Question
Exhibit
# show decryption statistics Decryption failures: 120 SSL handshake failures: 80 Certificate validation failures: 40 Decryption successful: 980
Refer to the exhibit. An administrator notices a high number of decryption failures. What is the most likely cause?
⚠ Common exam trap
Palo Alto Networks often tests the distinction between decryption failures caused by untrusted certificates versus configuration issues like cache size or protocol mismatches, trapping candidates who confuse performance problems with trust-related handshake failures.
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
✓
The firewall's certificate is not trusted by client devices.
When the firewall's certificate is not trusted by client devices, clients will reject the SSL handshake, resulting in decryption failures. This is a common issue in SSL Forward Proxy deployments where the firewall generates a certificate for each session, and clients must trust the firewall's CA certificate. Without this trust, clients display certificate warnings or fail to connect, leading to a high number of decryption failures.
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 SSL session cache size is too small.
Why it's wrong here
SSL session cache size affects performance, not certificate validation.
- ✓
The firewall's certificate is not trusted by client devices.
Why this is correct
Certificate validation failures indicate that clients cannot verify the firewall's certificate.
- ✗
SSL Forward Proxy is not enabled.
Why it's wrong here
Forward Proxy is already enabled because decryption statistics are shown.
- ✗
Non-HTTP traffic is being decrypted.
Why it's wrong here
Non-HTTP traffic would not cause certificate validation failures.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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