CAS-004 Security Engineering Practice Question
A security engineer is configuring a VPN between two sites and needs to ensure data confidentiality and integrity. Which IPsec mode and protocol combination should be used to encrypt the entire IP packet including the header?
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
✓
Tunnel mode with ESP
IPsec tunnel mode with ESP encrypts and authenticates the entire IP packet, providing confidentiality and integrity. Transport mode only encrypts the payload, and AH provides integrity without encryption.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Transport mode with ESP
Why it's wrong here
ESP encrypts the payload but transport mode leaves the original IP header intact and unencrypted, so the entire packet is not protected. Transport mode with ESP suits host-to-host traffic where the endpoints themselves are the tunnel and header concealment is not required.
- ✗
Transport mode with AH
Why it's wrong here
AH provides integrity but no encryption; transport mode does not encrypt the header.
- ✗
Tunnel mode with AH
Why it's wrong here
Tunnel mode encapsulates the whole original packet, yet AH still supplies no encryption, so the payload and headers travel in cleartext and confidentiality is unmet. AH in tunnel mode fits integrity-only site-to-site links where encryption is explicitly out of scope.
- ✓
Tunnel mode with ESP
Why this is correct
Tunnel mode encapsulates the complete original IP packet, header included, inside a new IP packet, so the original addresses are hidden. ESP provides confidentiality and integrity for that payload. Transport mode would leave the original header exposed, failing the stated requirement.
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.
About these practice questions
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CAS-005 practice question is part of Courseiva's free CompTIA 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 CAS-005 exam.