- A
Pre-shared key (PSK)
Why wrong: PSK is static and does not provide forward secrecy.
- B
RSA key exchange
Why wrong: RSA key exchange does not provide forward secrecy as the session key is encrypted with the server's static key.
- C
Ephemeral Diffie-Hellman (DHE or ECDHE)
Ephemeral key exchange ensures that session keys are temporary and not linked to long-term keys.
- D
Diffie-Hellman with static keys
Why wrong: Static DH does not provide forward secrecy.
Question 42 of 1,000
SSCP Cryptography Practice Question
This SSCP practice question tests your understanding of cryptography. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
PSK is static and does not provide forward secrecy.
- ✗
RSA key exchange
Why it's wrong here
RSA key exchange does not provide forward secrecy as the session key is encrypted with the server's static key.
- ✓
Ephemeral Diffie-Hellman (DHE or ECDHE)
Why this is correct
Ephemeral key exchange ensures that session keys are temporary and not linked to long-term keys.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Diffie-Hellman with static keys
Why it's wrong here
Static DH does not provide forward secrecy.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates 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.
Detailed technical explanation
How to think about this question
In IPsec, DHE (using Diffie-Hellman groups like 14, 19, or 20) generates ephemeral public/private pairs per IKE phase 1 or phase 2 exchange, ensuring that the derived shared secret is unique to that session. The server's long-term private key (e.g., RSA or ECDSA) is used only for authentication (signing the ephemeral public key), not for key agreement, so even if that long-term key is later stolen, the ephemeral keys are already discarded. This is why RFC 4301 (IPsec) recommends DHE for PFS, and it is commonly enforced in secure VPN deployments.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this SSCP question test?
Cryptography — This question tests Cryptography — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this SSCP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jul 4, 2026
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.
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