CAS-004 Security Engineering Practice Question
A security engineer is configuring IPsec VPN between two sites. The requirement is to encrypt the entire IP packet, including the original IP header. Which IPsec mode and protocol should be used?
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
Tunnel mode with ESP encrypts the entire original IP packet and adds a new IP header, which is required for gateway-to-gateway VPNs. ESP provides encryption and optional authentication.
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 AH
Why it's wrong here
Transport mode does not encrypt the original IP header; AH provides authentication but not encryption.
- ✗
Tunnel mode with AH
Why it's wrong here
AH does not provide encryption, only integrity.
- ✗
Transport mode with ESP
Why it's wrong here
Transport mode encrypts only the payload, not the original IP header.
- ✓
Tunnel mode with ESP
Why this is correct
Tunnel mode with ESP encrypts the entire original packet, including headers.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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