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CAS-004 Practice Question: Must select a cryptographic algorithm to ensure…
A security engineer must select a cryptographic algorithm to ensure non-repudiation for digitally signed documents. Which algorithm is most appropriate?
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
✓
RSA with SHA-256
RSA with SHA-256 (Option A) is the most appropriate choice because digital signatures rely on asymmetric cryptography. RSA provides the private key for signing and the public key for verification, ensuring that only the signer could have created the signature. SHA-256 provides a secure hash. Together, they provide non-repudiation. In contrast, HMAC-SHA256 (Option B) uses a symmetric key, which does not provide non-repudiation because both parties share the key. AES-256 in GCM mode (Option C) is a symmetric encryption algorithm, not a signature algorithm. Elliptic Curve Diffie-Hellman (ECDH) (Option D) is a key exchange protocol, not suitable for digital signatures.
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 with SHA-256
Why this is correct
RSA is an asymmetric algorithm, and combined with SHA-256, it produces a digital signature that binds the signer's identity, providing non-repudiation.
- ✗
HMAC-SHA256
Why it's wrong here
HMAC uses a shared secret, so it cannot provide non-repudiation; both parties can generate the same MAC.
- ✗
AES-256 in GCM mode
Why it's wrong here
AES is a symmetric cipher; it cannot provide non-repudiation because the key is shared.
- ✗
Elliptic Curve Diffie-Hellman (ECDH)
Why it's wrong here
ECDH is for key agreement, not digital signatures; it does not provide non-repudiation.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.