SSCP Cryptography Practice Question
A company is deploying a VPN using IPsec. They want to ensure that even if the private key of the server is compromised, past session keys cannot be derived. Which key exchange method should they use?
⚠ Common exam trap
It's easy for candidates to confuse authentication with key exchange, assuming that RSA or static DH provides PFS because they involve public-key cryptography, but only ephemeral DH ensures that session keys are not derived from long-term secrets.
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
✓
Ephemeral Diffie-Hellman (DHE or ECDHE)
Ephemeral Diffie-Hellman (DHE or ECDHE) provides perfect forward secrecy (PFS) because it generates a unique, temporary session key for each session using ephemeral key pairs. Even if the server's long-term private key is compromised, past session keys cannot be derived because they were created from ephemeral keys that are discarded after each session. This ensures that historical encrypted traffic remains secure.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Pre-shared key (PSK)
Why it's wrong here
A static pre-shared key provides no forward secrecy: compromise of the key exposes every session key derived from it, so past traffic becomes decryptable. PSK suits small, fixed peer sets where key distribution simplicity outweighs that risk, but it cannot meet this requirement.
- ✗
RSA key exchange
Why it's wrong here
RSA key exchange encrypts the session key under the server's long-term private key, so its compromise decrypts every recorded session, defeating forward secrecy. It is tempting because RSA is widely deployed for IPsec authentication and key transport, and is correct when forward secrecy is not required.
- ✓
Ephemeral Diffie-Hellman (DHE or ECDHE)
Why this is correct
Ephemeral Diffie-Hellman generates a fresh key pair per session, so the derived shared secret is never transmitted and cannot be recovered from the server's long-term private key. This satisfies the forward secrecy constraint: compromise of that key leaves previously negotiated session keys underivable.
- ✗
Diffie-Hellman with static keys
Why it's wrong here
Static Diffie-Hellman keys are long-term and reused, so compromise of the server's private key exposes the shared secret and therefore past session keys. It is tempting because Diffie-Hellman provides forward secrecy when ephemeral, which is the correct choice for this 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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SSCP practice question is part of Courseiva's free ISC2 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 SSCP exam.