easyMultiple Choice
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
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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 signatures with SHA-256 provide non-repudiation because only the holder of the private key can produce a signature that the corresponding public key verifies. This asymmetric property binds the signer to the document, unlike symmetric MACs where both parties share the secret.
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HMAC-SHA256
Why it's wrong here
HMAC-SHA256 uses a symmetric shared secret, so any holder of that key could produce the tag, leaving no way to prove which party signed. It is tempting because it verifies integrity and authenticity, and it would be correct for protecting API messages between two trusted systems.
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AES-256 in GCM mode
Why it's wrong here
AES-256-GCM provides symmetric confidentiality and integrity, but both parties share the key, so neither can be uniquely bound to the document. It is tempting because GCM authenticates ciphertext, and it would be correct for encrypting data at rest or TLS record payloads.
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Elliptic Curve Diffie-Hellman (ECDH)
Why it's wrong here
ECDH performs key agreement, producing a shared secret between two parties; it generates no signature, so a signer cannot be bound to a document. It is tempting because it is asymmetric and elliptic-curve based, and it would be correct for establishing a session key over an untrusted channel.
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.