This CAS-004 practice question tests your understanding of security engineering and cryptography. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A security engineer is selecting an asymmetric encryption algorithm for a system that must provide non-repudiation and long-term security (at least 20 years). The system has limited computational resources. Which of the following is the best choice?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
RSA 4096
Why wrong: RSA 4096 provides long-term security but requires more computation and larger keys.
ECDSA P-384
Why wrong: ECDSA P-384 is secure but slower and more complex than Ed25519.
Ed25519
Ed25519 uses elliptic curves with 128-bit security, small keys, and fast operations, suitable for long-term security.
DSA 3072
Why wrong: DSA is older and less efficient; not recommended for new systems.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Ed25519
Ed25519 offers high security with small key sizes and efficient computation, and is suitable for long-term security. RSA 4096 is slower and has larger keys. ECDSA P-384 is secure but slower than Ed25519. DSA is older and less efficient.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
RSA 4096
Why it's wrong here
RSA 4096 provides long-term security but requires more computation and larger keys.
ECDSA P-384
Why it's wrong here
ECDSA P-384 is secure but slower and more complex than Ed25519.
Ed25519
Why this is correct
Ed25519 uses elliptic curves with 128-bit security, small keys, and fast operations, suitable for long-term security.
Clue confirmation
The clue words "best", "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
DSA 3072
Why it's wrong here
DSA is older and less efficient; not recommended for new systems.
Common exam traps
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the CAS-004 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which CAS-004 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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FAQ
Security Engineering and Cryptography — This question tests Security Engineering and Cryptography — Read the scenario before looking for a memorised answer..
The correct answer is: Ed25519 — Ed25519 offers high security with small key sizes and efficient computation, and is suitable for long-term security. RSA 4096 is slower and has larger keys. ECDSA P-384 is secure but slower than Ed25519. DSA is older and less efficient.
Identify which CAS-004 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Yes — watch for: "best", "least". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
This CAS-004 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-004 exam.
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