SSCP Cryptography Practice Question
A security administrator is configuring a VPN gateway that must support perfect forward secrecy for IPsec connections. Which key establishment method should be enabled to ensure that compromise of a long-term key does not expose previously established session keys?
⚠ Common exam trap
The trap here is assuming that any strong authentication method, such as a PSK or RSA key transport, automatically provides perfect forward secrecy.
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
✓
Internet Key Exchange with ephemeral Diffie-Hellman (DHE)
Perfect forward secrecy requires that session keys cannot be recovered from a long-term secret. Ephemeral Diffie-Hellman achieves this by generating temporary key pairs for each session and deleting them after use. Static PSK and static RSA key transport reuse long-term secrets, so compromise of those secrets can expose past sessions. HMAC provides integrity, not key establishment, and is therefore not the correct control.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
RSA key transport with the gateway's static RSA key pair
Why it's wrong here
RSA key transport encrypts the session key with the gateway's long-term RSA public key. If the corresponding private key is later compromised, an attacker can decrypt recorded session keys, breaking forward secrecy. Static RSA key transport does not generate ephemeral secrets per session, so it fails the requirement.
- ✗
Pre-shared key authentication with a static PSK
Why it's wrong here
A static pre-shared key is a long-term secret reused across sessions. If it is compromised, an attacker who recorded past VPN traffic may be able to derive the same session keys and decrypt that traffic. PSK authentication alone does not provide perfect forward secrecy; it must be combined with an ephemeral key exchange such as DHE.
- ✗
HMAC-SHA-256 for integrity protection of IKE messages
Why it's wrong here
HMAC-SHA-256 is used to authenticate and integrity-protect IKE messages; it does not establish session keys or provide forward secrecy. While important for protecting the key exchange itself, it is not a key establishment method. The requirement is about deriving ephemeral session keys, which HMAC alone cannot accomplish.
- ✓
Internet Key Exchange with ephemeral Diffie-Hellman (DHE)
Why this is correct
Ephemeral Diffie-Hellman generates a fresh key pair for each session and discards it afterward, so compromise of the long-term authentication key does not reveal past session keys. This property is exactly what perfect forward secrecy requires. Enabling DHE in IKE allows the VPN gateway to derive unique session keys per connection, protecting previously recorded traffic.
Visual reference
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official ISC2 exam blueprint
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.