The answer is to modify the KMS key policy to allow kms:Decrypt only for the production IAM role and deny it for all other principals. This is the most secure approach because KMS key policies act as the ultimate access control for the customer master key (CMK); even if an IAM policy grants decrypt permissions, a restrictive key policy explicitly denying all other principals will override that access, ensuring only the designated role can decrypt the RDS instance. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of the critical distinction between IAM policies and resource-based KMS key policies, a common trap where candidates mistakenly believe IAM policies alone can restrict decryption when the key policy is left permissive. Remember the memory tip: "Key policy is king" — for KMS-controlled resources like RDS, the key policy is the final gatekeeper, not IAM.
SCS-C02 Data Protection Practice Question
This SCS-C02 practice question tests your understanding of data protection. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 company uses AWS KMS to encrypt data at rest in Amazon RDS for MySQL. The security team needs to ensure that the RDS instance can only be decrypted by a specific IAM role used by the production application, and not by any other IAM user or role. What is the most secure way to achieve this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Modify the KMS key policy to allow 'kms:Decrypt' only for the production role and deny for all other principals.
Option B is correct because using a KMS key policy that grants decrypt permission only to the production role ensures that only that role can decrypt the RDS instance. Option A is wrong because IAM policies alone cannot restrict decryption if the key policy allows broader access. Option C is wrong because RDS does not support attaching IAM roles at the instance level for this purpose. Option D is wrong because it makes the key accessible to all users.
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.
✗
Enable key rotation and use a custom key store to isolate the key.
Why it's wrong here
Key rotation or custom key store does not restrict decrypt permissions.
✗
Create an IAM policy that denies 'kms:Decrypt' to all principals except the production role, and attach it to the key.
Why it's wrong here
IAM policies cannot be attached to KMS keys; key policies are used for KMS.
✗
Attach the production IAM role to the RDS instance and use a condition in the IAM role's policy to allow decrypt.
Why it's wrong here
RDS does not support attaching IAM roles to the DB instance for encryption operations.
✓
Modify the KMS key policy to allow 'kms:Decrypt' only for the production role and deny for all other principals.
Why this is correct
Key policies directly control access to the KMS key.
Related concept
Standard ACLs match source addresses.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 SCS-C02 ACL questions on filtering logic and placement.
Data Protection — This question tests Data Protection — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Modify the KMS key policy to allow 'kms:Decrypt' only for the production role and deny for all other principals. — Option B is correct because using a KMS key policy that grants decrypt permission only to the production role ensures that only that role can decrypt the RDS instance. Option A is wrong because IAM policies alone cannot restrict decryption if the key policy allows broader access. Option C is wrong because RDS does not support attaching IAM roles at the instance level for this purpose. Option D is wrong because it makes the key accessible to all users.
What should I do if I get this SCS-C02 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 SCS-C02 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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