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
Transport mode encrypts only the payload, leaving the original header in plaintext.
- ✗
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
AH provides integrity but no encryption, so confidentiality is not assured.
- ✓
Tunnel mode with ESP
Why this is correct
ESP in tunnel mode encrypts the entire IP packet, including the original header.
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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