CAS-004 Security Engineering Practice Question
An organization is deploying a new IoT device that must securely update its firmware over the air (OTA). The device has limited processing power and memory. Which cryptographic solution would provide the BEST balance of security and performance for verifying firmware updates?
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
✓
Ed25519 digital signatures
Ed25519 is a fast and secure digital signature algorithm that performs well on constrained devices. RSA 4096 is computationally expensive. HMAC-SHA256 is a symmetric key technique and requires key management overhead. AES-256-GCM is for encryption, not verification.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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RSA-4096 digital signatures
Why it's wrong here
RSA-4096 verification demands large modular exponentiation and key storage, exceeding the constrained device's processing and memory budget. It is tempting because RSA signatures are widely trusted, but RSA would be the correct choice where ample compute exists and legacy compatibility outweighs efficiency.
- ✓
Ed25519 digital signatures
Why this is correct
Ed25519 signatures verify firmware authenticity with minimal computation, satisfying the constrained processor and memory requirement. Its compact 64-byte signatures and 32-byte keys reduce storage and transmission overhead, while verification is far faster than RSA at equivalent security. This makes it ideal for OTA updates on resource-limited IoT devices.
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HMAC-SHA256 with pre-shared key
Why it's wrong here
HMAC-SHA256 with a pre-shared key verifies integrity and authenticity but cannot provide non-repudiation, and every device holding the same key means one compromise forges updates fleet-wide. It is tempting because HMAC is fast and lightweight, and would suit closed environments where a single trusted party signs and distributes firmware.
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AES-256-GCM for authentication
Why it's wrong here
AES-256-GCM is a symmetric cipher providing confidentiality and authentication, but the verifier must hold the same secret key as the signer, so any device can forge firmware. It is tempting because GCM is fast and hardware-accelerated, and would be correct for encrypting firmware payloads in transit rather than verifying publisher authenticity.
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.