SAA-C03 Design Secure Architectures Practice Question
This SAA-C03 practice question tests your understanding of design secure architectures. 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.
Based on the exhibit, a partner account uploads encrypted objects to a central S3 bucket and later reads them back. The S3 permissions are correct, but the requests still fail. What change is required so the partner workload can use the customer-managed KMS key safely?
Replace SSE-KMS with S3 object ACLs so the partner account can bypass KMS authorization.
Why wrong: ACLs control S3 object access, not KMS cryptographic authorization. The error is coming from KMS decrypt operations, so ACL changes would not solve it.
B
Create a new bucket in the partner account and copy the objects there to avoid cross-account encryption.
Why wrong: Moving the data would avoid the immediate trust problem, but it is an architectural workaround, not the required fix. It adds duplication, cost, and operational complexity without addressing the KMS sharing requirement.
C
Switch the bucket to SSE-S3 so the partner role no longer needs KMS permissions.
Why wrong: SSE-S3 removes customer-managed key control, which may violate security or compliance requirements. The scenario requires safe use of the customer-managed KMS key, not eliminating it.
D
Update the CMK key policy, or add a tightly scoped grant, to allow the partner role the required KMS actions through S3.
Cross-account access to SSE-KMS encrypted objects requires KMS authorization in addition to S3 authorization. The key policy must trust the partner role, and the permissions should be limited to the needed KMS actions such as Decrypt, Encrypt, and GenerateDataKey with a service condition for S3. That is why the partner can have valid S3 permissions and still fail until the KMS policy is fixed.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Update the CMK key policy, or add a tightly scoped grant, to allow the partner role the required KMS actions through S3.
The correct answer is D because when using a customer-managed KMS key (CMK) for SSE-KMS in a cross-account scenario, the key policy must explicitly grant the partner account's IAM role the necessary KMS actions (kms:Decrypt, kms:GenerateDataKey) to allow S3 to perform the encryption/decryption on behalf of the partner. Without this policy update or a tightly scoped grant, S3 cannot authorize the KMS operation even if the S3 bucket policy permits the upload/read.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Replace SSE-KMS with S3 object ACLs so the partner account can bypass KMS authorization.
Why it's wrong here
ACLs control S3 object access, not KMS cryptographic authorization. The error is coming from KMS decrypt operations, so ACL changes would not solve it.
✗
Create a new bucket in the partner account and copy the objects there to avoid cross-account encryption.
Why it's wrong here
Moving the data would avoid the immediate trust problem, but it is an architectural workaround, not the required fix. It adds duplication, cost, and operational complexity without addressing the KMS sharing requirement.
✗
Switch the bucket to SSE-S3 so the partner role no longer needs KMS permissions.
Why it's wrong here
SSE-S3 removes customer-managed key control, which may violate security or compliance requirements. The scenario requires safe use of the customer-managed KMS key, not eliminating it.
✓
Update the CMK key policy, or add a tightly scoped grant, to allow the partner role the required KMS actions through S3.
Why this is correct
Cross-account access to SSE-KMS encrypted objects requires KMS authorization in addition to S3 authorization. The key policy must trust the partner role, and the permissions should be limited to the needed KMS actions such as Decrypt, Encrypt, and GenerateDataKey with a service condition for S3. That is why the partner can have valid S3 permissions and still fail until the KMS policy is fixed.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the S3 bucket policy alone is sufficient for cross-account access with SSE-KMS, forgetting that KMS requires its own separate authorization via the key policy or a grant, which is a frequent point of failure in multi-account architectures.
Trap categories for this question
Scenario analysis trap
SSE-S3 removes customer-managed key control, which may violate security or compliance requirements. The scenario requires safe use of the customer-managed KMS key, not eliminating it.
Detailed technical explanation
How to think about this question
Under the hood, when S3 receives a request to upload or read an object encrypted with SSE-KMS, it calls KMS on behalf of the requester using the requester's IAM permissions. For cross-account access, the CMK key policy must include a statement allowing the external IAM role to perform kms:GenerateDataKey (for upload) and kms:Decrypt (for read), and the role must also have the corresponding KMS permissions in its own IAM policy. A common real-world scenario is a centralized logging bucket where multiple accounts send encrypted logs; the key policy must list each account's role ARN explicitly.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Design Secure Architectures — This question tests Design Secure Architectures — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Update the CMK key policy, or add a tightly scoped grant, to allow the partner role the required KMS actions through S3. — The correct answer is D because when using a customer-managed KMS key (CMK) for SSE-KMS in a cross-account scenario, the key policy must explicitly grant the partner account's IAM role the necessary KMS actions (kms:Decrypt, kms:GenerateDataKey) to allow S3 to perform the encryption/decryption on behalf of the partner. Without this policy update or a tightly scoped grant, S3 cannot authorize the KMS operation even if the S3 bucket policy permits the upload/read.
What should I do if I get this SAA-C03 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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