The answer is to update the CMK key policy or add a tightly scoped grant for the partner role. This is required because when using SSE-KMS with a customer-managed key in a cross-account S3 setup, the KMS key policy acts as the primary access control—S3 bucket policies alone cannot authorize KMS actions. Even with correct S3 permissions, the partner workload fails because KMS does not trust the partner account’s IAM role unless the key policy explicitly allows kms:Decrypt and kms:GenerateDataKey for that role, enabling S3 to perform encryption and decryption on its behalf. On the SAA-C03 exam, this scenario tests your understanding that cross-account KMS access requires a resource-based policy on the key, not just an identity-based policy on the role. A common trap is assuming the bucket policy is sufficient, but KMS enforces its own authorization layer. Memory tip: “Key policy first, bucket policy second—KMS is the gatekeeper for SSE-KMS.”
SAA-C03 Design Secure Architectures Practice Question
This SAA-C03 practice question tests your understanding of design secure architectures. 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. A key principle to apply: sSE-KMS requires both S3 object permissions and KMS key permissions for access.. 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.
Option D is correct 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, the KMS service will deny the request even if S3 bucket policies are correctly configured.
Key principle: SSE-KMS requires both S3 object permissions and KMS key permissions for access.
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
SSE-KMS requires both S3 object permissions and KMS key permissions for access.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume S3 bucket policies alone control all access, forgetting that SSE-KMS introduces a separate authorization layer at KMS that requires explicit cross-account permissions in the key policy.
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 credentials. For cross-account access, the CMK key policy must include a statement allowing the external principal (the partner role) to perform kms:Decrypt and kms:GenerateDataKey, and the partner account's IAM policy must also grant those actions. A common real-world scenario is a data lake where multiple accounts contribute encrypted data; without proper key policy configuration, all cross-account operations fail with an 'Access Denied' error from KMS, not S3.
KKey Concepts to Remember
SSE-KMS requires both S3 object permissions and KMS key permissions for access.
KMS key policies define who can use the CMK and for which cryptographic operations.
Cross-account access to SSE-KMS objects needs explicit KMS key policy grants for the external principal.
KMS grants offer a more granular, temporary, or condition-based alternative to key policy statements.
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
SSE-KMS requires both S3 object permissions and KMS key permissions for access.
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.
Review sSE-KMS requires both S3 object permissions and KMS key permissions for access., then practise related SAA-C03 questions on the same topic to reinforce the concept.
Design Secure Architectures — This question tests Design Secure Architectures — SSE-KMS requires both S3 object permissions and KMS key permissions for access..
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. — Option D is correct 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, the KMS service will deny the request even if S3 bucket policies are correctly configured.
What should I do if I get this SAA-C03 question wrong?
Review sSE-KMS requires both S3 object permissions and KMS key permissions for access., then practise related SAA-C03 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
SSE-KMS requires both S3 object permissions and KMS key permissions for access.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
hard
A.Replace SSE-KMS with S3 object ACLs so the partner account can bypass KMS authorization.
B.Create a new bucket in the partner account and copy the objects there to avoid cross-account encryption.
C.Switch the bucket to SSE-S3 so the partner role no longer needs KMS permissions.
✓ D.Update the CMK key policy, or add a tightly scoped grant, to allow the partner role the required KMS actions through S3.
Why D: 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.
Last reviewed: Jun 11, 2026
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