CAS-004 Security Engineering Practice Question
A security engineer is configuring SSH for a jump host used to access critical servers. The engineer wants to restrict the cryptographic algorithms to the most secure options. Which of the following should be DISABLED?
⚠ Common exam trap
CAS-005 often tests whether candidates can distinguish deprecated hash-based algorithms (SHA-1 kex) from still-secure primitives (AES-256, Ed25519, HMAC-SHA2) — the trap is picking a strong cipher or MAC thinking it's weak because it sounds old.
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
✓
Diffie-Hellman group-exchange with SHA-1
Diffie-Hellman group exchange with SHA-1 uses the SHA-1 hash for key exchange integrity, and SHA-1 is cryptographically broken (collision attacks demonstrated) and deprecated by NIST. For a hardened jump host, this kex algorithm should be disabled in favor of SHA-2-based groups like diffie-hellman-group-exchange-sha256 or curve25519-sha256.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Diffie-Hellman group-exchange with SHA-1
Why this is correct
Diffie-Hellman group exchange with SHA-1 uses a deprecated hash for key exchange integrity, weakening SSH against collision-based attacks. Disabling it enforces stronger SHA-2-based key exchange algorithms, meeting the requirement to restrict the jump host to the most secure options.
- ✗
AES-256-CTR
Why it's wrong here
AES-256-CTR is a symmetric cipher for bulk data encryption, not an SSH key-exchange or MAC algorithm, so disabling it removes confidentiality without hardening negotiation. It is tempting because 256-bit AES sounds weak next to ChaCha20-Poly1305, but CTR mode remains secure; the correct answer targets legacy algorithms such as ssh-rsa or 3DES-cbc.
- ✗
Ed25519 for host keys
Why it's wrong here
Ed25519 is a modern elliptic-curve signature algorithm and among the strongest host-key options, so disabling it weakens rather than hardens the jump host. It is tempting because older RSA and DSA keys are the ones to remove. Disable ssh-dss and SHA-1 based algorithms instead, retaining Ed25519.
- ✗
HMAC-SHA2-256
Why it's wrong here
HMAC-SHA2-256 is a current, secure message authentication code and should remain enabled; disabling it removes integrity protection without security gain. It is tempting because SHA-1 variants such as hmac-sha1 are deprecated. Disable hmac-sha1, hmac-md5 and similar legacy MACs, keeping the SHA-2 family.
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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About these practice questions
This CAS-005 question is part of Courseiva's 973-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
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 CompTIA exam blueprint
This CAS-005 practice question is part of Courseiva's free CompTIA 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 CAS-005 exam.