CAS-004 Security Engineering and Cryptography Practice Question
This CAS-004 practice question tests your understanding of security engineering and cryptography. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 administrator is hardening SSH access to a jump host. The requirement is to allow only key-based authentication and restrict the use of weak cryptographic algorithms. Which of the following configurations accomplishes this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Set PubkeyAuthentication yes, PasswordAuthentication no, and configure Ciphers and MACs to strong algorithms only
Setting PubkeyAuthentication yes and PasswordAuthentication no enforces key-only access. Disabling weak ciphers and MACs is also needed. Option A describes the correct approach. Option B allows passwords. Option C allows passwords with keyboard-interactive. Option D allows passwords.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Set PermitRootLogin prohibit-password and PasswordAuthentication yes
Why it's wrong here
Password authentication is allowed.
✓
Set PubkeyAuthentication yes, PasswordAuthentication no, and configure Ciphers and MACs to strong algorithms only
Why this is correct
This enforces key-based auth and removes weak ciphers/MACs.
Related concept
Standard ACLs match source addresses.
✗
Set PasswordAuthentication yes and use a strong password policy
Why it's wrong here
Passwords are allowed, which is against the requirement.
✗
Set AuthenticationMethods publickey,keyboard-interactive
Why it's wrong here
Keyboard-interactive may allow passwords.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CAS-004 ACL questions on filtering logic and placement.
Security Engineering and Cryptography — This question tests Security Engineering and Cryptography — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Set PubkeyAuthentication yes, PasswordAuthentication no, and configure Ciphers and MACs to strong algorithms only — Setting PubkeyAuthentication yes and PasswordAuthentication no enforces key-only access. Disabling weak ciphers and MACs is also needed. Option A describes the correct approach. Option B allows passwords. Option C allows passwords with keyboard-interactive. Option D allows passwords.
What should I do if I get this CAS-004 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CAS-004 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Question Discussion
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