- 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.
Quick Answer
The answer is to update Company A’s KMS key policy to explicitly grant the partner role in Company B the kms:Encrypt, kms:GenerateDataKey, and kms:DescribeKey actions, and then add a matching IAM policy in Company B that allows those same actions on Company A’s key ARN. This is correct because cross-account SSE-KMS S3 uploads require a two-way authorization: the KMS key policy must allow the external principal, and the external principal’s IAM policy must permit the KMS actions—neither the S3 bucket policy nor the IAM policy alone can authorize KMS operations, as KMS key policies are the primary access control for customer-managed keys. On the SAA-C03 exam, this scenario tests your understanding that KMS key policies act as a resource-based firewall for cross-account access, and a common trap is assuming the bucket policy or the partner’s IAM role is sufficient without modifying the key policy. A reliable memory tip is “key policy first, IAM second”—the key policy must explicitly name the external role before any IAM permissions take effect, and always constrain with kms:ViaService or EncryptionContext conditions for security.
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.
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.
Option A is correct because cross-account SSE-KMS uploads require both the KMS key policy in Company A to explicitly grant the partner role principal the necessary KMS actions (kms:Encrypt, kms:GenerateDataKey, kms:DescribeKey) and an IAM policy in Company B that allows the partner role to call those actions on Company A's key ARN. The bucket policy alone cannot authorize KMS operations; KMS key policies act as the primary access control for customer-managed keys, and without the key policy grant, the partner role's IAM permissions are insufficient. Constraining the IAM policy to the target S3 bucket context (using kms:ViaService or kms:EncryptionContext conditions) adds a security best practice by limiting the key's use to only that specific S3 bucket.
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.
- ✓
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
Read the scenario before looking for a memorised answer.
- ✗
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 is sufficient for cross-account KMS access, overlooking that KMS key policies are the mandatory gatekeeper for external principals, and that the key policy must explicitly grant the external role.
Detailed technical explanation
How to think about this question
Under the hood, KMS uses a resource-based policy (key policy) that is separate from IAM policies; for cross-account access, the key policy must explicitly grant the external principal, and the external principal must have an IAM policy allowing the same actions. The kms:ViaService condition key can restrict KMS actions to requests that originate from a specific AWS service (e.g., s3.amazonaws.com), and kms:EncryptionContext can further limit use to a specific bucket ARN, providing granular security. In real-world scenarios, failing to include kms:DescribeKey in the key policy can cause uploads to fail because the S3 service calls DescribeKey to validate the key before encrypting.
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
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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 — Read the scenario before looking for a memorised answer..
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. — Option A is correct because cross-account SSE-KMS uploads require both the KMS key policy in Company A to explicitly grant the partner role principal the necessary KMS actions (kms:Encrypt, kms:GenerateDataKey, kms:DescribeKey) and an IAM policy in Company B that allows the partner role to call those actions on Company A's key ARN. The bucket policy alone cannot authorize KMS operations; KMS key policies act as the primary access control for customer-managed keys, and without the key policy grant, the partner role's IAM permissions are insufficient. Constraining the IAM policy to the target S3 bucket context (using kms:ViaService or kms:EncryptionContext conditions) adds a security best practice by limiting the key's use to only that specific S3 bucket.
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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Same concept, more angles
1 more ways this is tested on SAA-C03
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. 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?
medium- ✓ 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.
- B.In Company B's IAM policy, allow kms:Encrypt on Company A's KMS key ARN, without changing Company A's key policy.
- C.Create a new KMS key in Company B and configure Company A's S3 bucket to use that key for SSE-KMS.
- 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 A: 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.
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Last reviewed: Jun 11, 2026
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