CAS-004 Security Engineering Practice Question
During a security assessment, a penetration tester discovers that a smart card used for authentication is vulnerable to a timing attack. The card uses a cryptographic algorithm that has data-dependent timing variations. Which of the following algorithms is MOST likely being used on the smart card?
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 Chinese Remainder Theorem
RSA decryption (using CRT) is known to be vulnerable to timing attacks if not implemented with constant-time functions. AES is typically resistant if implemented correctly. SHA-256 is a hash function, not used for signing/decryption. ECDSA can be vulnerable to timing attacks if not constant-time, but RSA is more commonly associated with classic timing vulnerabilities.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ECDSA P-384
Why it's wrong here
While ECDSA can be vulnerable to timing attacks, modern implementations are often constant-time, and RSA is the classic example.
- ✗
SHA-256
Why it's wrong here
SHA-256 is a hash function and typically not used in a way that would leak key material via timing.
- ✗
AES-256-CBC
Why it's wrong here
AES implementations are usually constant-time, and CBC mode does not directly expose timing variations.
- ✓
RSA with Chinese Remainder Theorem
Why this is correct
RSA-CRT implementations often have timing variations that can be exploited to recover the private key.
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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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.