SSCP Cryptography Practice Question
A company deploys a VPN gateway that uses Diffie-Hellman key exchange to establish session keys. A security auditor warns that the gateway is vulnerable to a man-in-the-middle attack during key agreement. Which of the following should the administrator implement to mitigate this risk?
⚠ Common exam trap
The trap here is assuming that increasing key size or enabling perfect forward secrecy provides authentication, when these properties are independent of identity verification.
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
✓
Deploy certificates and use authenticated Diffie-Hellman
The vulnerability exists because unauthenticated Diffie-Hellman allows an attacker to impersonate each party. Using certificates to authenticate the exchange ensures that each side verifies the other's identity before deriving session keys. Larger groups, perfect forward secrecy, or static keys do not by themselves prevent impersonation, so authenticated Diffie-Hellman is the required mitigation.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the Diffie-Hellman group to 8192-bit MODP
Why it's wrong here
Increasing the Diffie-Hellman group size strengthens resistance to discrete logarithm attacks but does not authenticate the parties. An unauthenticated Diffie-Hellman exchange remains vulnerable to a man-in-the-middle who can establish separate keys with each side, so a larger group alone does not mitigate the described risk.
- ✗
Configure the gateway to use static Diffie-Hellman keys
Why it's wrong here
Static Diffie-Hellman keys can provide implicit authentication if the public keys are trusted, but they lack forward secrecy and are rarely used in modern VPNs. Simply configuring static keys without a trusted distribution mechanism does not guarantee authentication and may worsen key compromise impact.
- ✓
Deploy certificates and use authenticated Diffie-Hellman
Why this is correct
Authenticated Diffie-Hellman binds the exchange to verified identities using digital certificates, preventing an attacker from substituting keys. By validating each peer's certificate, the VPN gateway ensures it shares a key only with the legitimate party, directly mitigating the man-in-the-middle risk described by the auditor.
- ✗
Enable perfect forward secrecy on the VPN gateway
Why it's wrong here
Perfect forward secrecy ensures that compromise of long-term keys does not expose past session keys, but it does not authenticate the key exchange. Without authentication, an attacker can still perform a man-in-the-middle attack during the exchange, so enabling PFS does not address the stated vulnerability.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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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.