CAS-004 Security Engineering Practice Question
A company is migrating its internal services to use SSH key-based authentication instead of passwords. The security policy requires using the strongest supported algorithms. The SSH server supports the following key exchange algorithms: diffie-hellman-group14-sha256, ecdh-sha2-nistp384, curve25519-sha256. Which algorithm should the administrator choose to meet the policy?
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
✓
curve25519-sha256
Curve25519 (X25519) is the recommended elliptic curve for key exchange due to its strong security and performance advantages. It is considered more secure than NIST curves and traditional Diffie-Hellman.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ecdh-sha2-nistp384
Why it's wrong here
NIST P-384 is strong, but curve25519 is generally preferred for its security and performance.
- ✗
diffie-hellman-group14-sha256
Why it's wrong here
This is a 2048-bit Diffie-Hellman group, which is weaker than elliptic curve options.
- ✗
All three are equally strong
Why it's wrong here
They have different security levels; curve25519 is considered the strongest.
- ✓
curve25519-sha256
Why this is correct
X25519 is widely recommended for its security and efficiency.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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