PCNSE Decryption and SSL Inspection Practice Question
During SSL decryption, the firewall logs show 'ssl_decrypt_unsupported_cipher' errors for several connections. What is the likely cause and solution?
⚠ Common exam trap
It's easy for candidates to confuse cipher suite mismatch with certificate trust issues or rule misconfiguration, but the specific error message 'ssl_decrypt_unsupported_cipher' directly points to the cipher suite list in the SSL/TLS service profile.
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 SSL/TLS service profile does not include the cipher suites used by the client or server.
The 'ssl_decrypt_unsupported_cipher' error indicates that the firewall's SSL/TLS proxy cannot negotiate a common cipher suite with the client or server during the decryption handshake. This occurs when the cipher suites configured in the firewall's SSL/TLS service profile do not include the ciphers offered by the client or required by the server. The solution is to update the service profile to include the necessary cipher suites, such as those based on AES-GCM or CHACHA20-POLY1305, ensuring compatibility.
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 SSL/TLS service profile does not include the cipher suites used by the client or server.
Why this is correct
The error indicates a cipher suite mismatch: the negotiated algorithm is absent from the firewall's SSL/TLS service profile. Adding the required cipher suites to that profile, or aligning it with the endpoints' supported suites, resolves the failed handshakes.
- ✗
The firewall does not support decryption of that traffic.
Why it's wrong here
The firewall does support SSL decryption generally; the error names a specific cipher suite it cannot process, so the fix is adjusting the decryption profile's allowed ciphers or excluding that traffic. It tempts as a broad explanation, but it is correct only when decryption is entirely unavailable for a protocol.
- ✗
The decryption certificate is not trusted by the client.
Why it's wrong here
A client trust problem produces certificate-validation failures, not an unsupported-cipher error, which concerns the cipher suite negotiated. It tempts because untrusted certificates are a common decryption fault, but the correct fix there is distributing the forward-trust CA, not addressing cipher support.
- ✗
The decryption rule is not matching the traffic.
Why it's wrong here
A rule mismatch means traffic bypasses decryption silently, producing no decryption log entry at all rather than a cipher error. It tempts because rule ordering is a frequent misconfiguration, but it is the right diagnosis when sessions appear as undecrypted, not when unsupported-cipher messages are logged.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNSE practice question is part of Courseiva's free Palo Alto Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNSE exam.