- A
In Company A's KMS key policy, allow Company B's partner role principal to use the key for kms:Encrypt, kms:GenerateDataKey, and kms:DescribeKey, and also add a matching IAM policy in Company B that grants the partner role those same KMS actions on Company A's key ARN, constrained to the target S3 bucket context when possible.
Cross-account SSE-KMS requires both the KMS key policy in the key owner account and an IAM policy in the caller account to allow the required KMS actions. Scoping the permissions to the specific bucket or encryption context reduces blast radius.
- B
In Company B's IAM policy, allow kms:Encrypt on Company A's KMS key ARN, without changing Company A's key policy.
Why wrong: KMS key policies govern access to the key; adding permissions in the caller account alone cannot override a restrictive key policy in the key owner account.
- C
Create a new KMS key in Company B and configure Company A's S3 bucket to use that key for SSE-KMS.
Why wrong: Switching bucket ownership encryption to a partner key changes trust boundaries and operational ownership. It also doesn't directly resolve the original denial and can complicate key administration.
- D
Disable key policy restrictions by setting the KMS key to enabled and removing all policy statements so that encryption automatically works for any principal.
Why wrong: Removing restrictions or effectively allowing all principals defeats security objectives and is not required for cross-account access. Key policies should be explicit and least-privilege.
SAA-C03 Design Secure Architectures Practice Question
This SAA-C03 practice question tests your understanding of design secure architectures. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. A key principle to apply: cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy.. 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.
Company A stores encrypted log files in its S3 bucket using SSE-KMS with a customer-managed KMS key. A partner application in Company B uploads objects into Company A's bucket using an IAM role in Company B. Uploads fail with an error indicating KMS access is denied (kms:Encrypt not authorized). Neither the partner IAM policy nor the S3 bucket policy currently mentions KMS.
What is the most secure and correct change to allow cross-account uploads to succeed?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
In Company A's KMS key policy, allow Company B's partner role principal to use the key for kms:Encrypt, kms:GenerateDataKey, and kms:DescribeKey, and also add a matching IAM policy in Company B that grants the partner role those same KMS actions on Company A's key ARN, constrained to the target S3 bucket context when possible.
For cross-account SSE-KMS uploads, the KMS key policy must explicitly grant the external IAM role principal the required KMS actions (kms:Encrypt, kms:GenerateDataKey, and kms:DescribeKey). Additionally, the partner account's IAM policy must also allow those same actions on the key ARN. This dual-permission model is required because KMS does not implicitly trust IAM policies in the key owner's account for cross-account access; the key policy is the authoritative gatekeeper. Option A correctly implements both sides, and constraining to the target S3 bucket context (via kms:ViaService or kms:EncryptionContext) adds a security best practice.
Key principle: Cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
In Company A's KMS key policy, allow Company B's partner role principal to use the key for kms:Encrypt, kms:GenerateDataKey, and kms:DescribeKey, and also add a matching IAM policy in Company B that grants the partner role those same KMS actions on Company A's key ARN, constrained to the target S3 bucket context when possible.
Why this is correct
Cross-account SSE-KMS requires both the KMS key policy in the key owner account and an IAM policy in the caller account to allow the required KMS actions. Scoping the permissions to the specific bucket or encryption context reduces blast radius.
Related concept
Cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy.
- ✗
In Company B's IAM policy, allow kms:Encrypt on Company A's KMS key ARN, without changing Company A's key policy.
Why it's wrong here
KMS key policies govern access to the key; adding permissions in the caller account alone cannot override a restrictive key policy in the key owner account.
- ✗
Create a new KMS key in Company B and configure Company A's S3 bucket to use that key for SSE-KMS.
Why it's wrong here
Switching bucket ownership encryption to a partner key changes trust boundaries and operational ownership. It also doesn't directly resolve the original denial and can complicate key administration.
- ✗
Disable key policy restrictions by setting the KMS key to enabled and removing all policy statements so that encryption automatically works for any principal.
Why it's wrong here
Removing restrictions or effectively allowing all principals defeats security objectives and is not required for cross-account access. Key policies should be explicit and least-privilege.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume an IAM policy in the partner account alone is sufficient for cross-account KMS access, forgetting that KMS key policies are the definitive authorization mechanism for external principals.
Detailed technical explanation
How to think about this question
Under the hood, KMS uses a resource-based policy (key policy) that must explicitly list external principals for cross-account access. The IAM policy in the external account then acts as a second gate—both must allow the action. The kms:EncryptionContext condition can be used to restrict the key usage to specific S3 bucket operations, preventing the partner from using the key for other purposes. In practice, this dual-policy pattern is also required for other KMS-integrated services like Lambda or EBS when accessed cross-account.
KKey Concepts to Remember
- Cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy.
- The KMS key policy is the ultimate authority for cross-account access to a KMS key.
- Required KMS actions for upload include `kms:Encrypt` and `kms:GenerateDataKey`.
- Applying resource-level conditions (e.g., `s3:x-amz-server-side-encryption-aws-kms-key-id`) enhances security.
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
Cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy.
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.
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FAQ
Questions learners often ask
What does this SAA-C03 question test?
Design Secure Architectures — This question tests Design Secure Architectures — Cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy..
What is the correct answer to this question?
The correct answer is: In Company A's KMS key policy, allow Company B's partner role principal to use the key for kms:Encrypt, kms:GenerateDataKey, and kms:DescribeKey, and also add a matching IAM policy in Company B that grants the partner role those same KMS actions on Company A's key ARN, constrained to the target S3 bucket context when possible. — For cross-account SSE-KMS uploads, the KMS key policy must explicitly grant the external IAM role principal the required KMS actions (kms:Encrypt, kms:GenerateDataKey, and kms:DescribeKey). Additionally, the partner account's IAM policy must also allow those same actions on the key ARN. This dual-permission model is required because KMS does not implicitly trust IAM policies in the key owner's account for cross-account access; the key policy is the authoritative gatekeeper. Option A correctly implements both sides, and constraining to the target S3 bucket context (via kms:ViaService or kms:EncryptionContext) adds a security best practice.
What should I do if I get this SAA-C03 question wrong?
Review cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy., then practise related SAA-C03 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Cross-account SSE-KMS requires permissions in both the key owner's KMS key policy and the caller's IAM policy.
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Last reviewed: Jun 11, 2026
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