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CAS-004 Practice Question: A security architect is designing a VPN solution…
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?
⚠ Common exam trap
It's easy for candidates to assume ESP always requires encryption, but ESP_NULL (RFC 2410) explicitly allows authentication-only mode, making it ideal when confidentiality is not needed.
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
✓
ESP in tunnel mode with null encryption
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.
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.
- ✗
AH in transport mode
Why it's wrong here
AH provides integrity and authentication but not encryption; cannot be used with null encryption.
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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