- A
Update the IAM role in account B to include kms:Encrypt and kms:GenerateDataKey; then kms:Decrypt will start working automatically.
Why wrong: kms:Encrypt and kms:GenerateDataKey do not grant kms:Decrypt authorization. The error indicates that the KMS key policy is denying the decrypt call, so the fix must adjust KMS key policy (and/or grants), not add unrelated IAM KMS actions.
- B
Update the KMS key policy in account A to allow the account B role principal to use kms:Decrypt on the key.
Cross-account use of a CMK requires the KMS key policy (in the CMK’s account) to allow the external principal to perform kms:Decrypt. Since CloudTrail shows the denial is by key policy, updating the key policy to grant the account B role kms:Decrypt on the specific key is the correct and least-privilege solution.
- C
Disable key policy for the CMK by switching to S3-managed encryption, because KMS key policies are always enforced regardless of grants.
Why wrong: Switching encryption types changes the encryption configuration and does not address the underlying requirement for authorization to use the specific CMK. Additionally, KMS key policy is not a concept that can be “disabled” for a CMK; the correct fix is to modify the CMK key policy (and/or create appropriate grants).
- D
Create an SCP in account A that allows kms:Decrypt for all accounts, avoiding changes to the key policy.
Why wrong: Service Control Policies (SCPs) are organization-level guardrails. Even if an SCP allows kms:Decrypt, KMS can still deny the call if the CMK’s key policy does not allow it. SCPs also do not replace the CMK key policy requirement for cross-account authorization.
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.
Account B has an IAM role that includes kms:Decrypt for a specific KMS key ARN in account A. However, when the role tries to read an S3 object encrypted with that CMK, the application fails with AccessDenied: not authorized to perform kms:Decrypt. CloudTrail shows the KMS API call is denied by key policy. What is the most secure and correct fix?
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
Update the KMS key policy in account A to allow the account B role principal to use kms:Decrypt on the key.
Option B is correct because cross-account access to a customer managed KMS key (CMK) requires the key policy to explicitly grant the external IAM role principal the necessary permissions (e.g., kms:Decrypt). Even if the IAM role in Account B has an IAM policy allowing kms:Decrypt, the KMS key policy in Account A acts as a resource-based policy that must also allow the action; without this, the request is denied by the key policy, as shown in CloudTrail.
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.
- ✗
Update the IAM role in account B to include kms:Encrypt and kms:GenerateDataKey; then kms:Decrypt will start working automatically.
Why it's wrong here
kms:Encrypt and kms:GenerateDataKey do not grant kms:Decrypt authorization. The error indicates that the KMS key policy is denying the decrypt call, so the fix must adjust KMS key policy (and/or grants), not add unrelated IAM KMS actions.
- ✓
Update the KMS key policy in account A to allow the account B role principal to use kms:Decrypt on the key.
Why this is correct
Cross-account use of a CMK requires the KMS key policy (in the CMK’s account) to allow the external principal to perform kms:Decrypt. Since CloudTrail shows the denial is by key policy, updating the key policy to grant the account B role kms:Decrypt on the specific key is the correct and least-privilege solution.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Disable key policy for the CMK by switching to S3-managed encryption, because KMS key policies are always enforced regardless of grants.
Why it's wrong here
Switching encryption types changes the encryption configuration and does not address the underlying requirement for authorization to use the specific CMK. Additionally, KMS key policy is not a concept that can be “disabled” for a CMK; the correct fix is to modify the CMK key policy (and/or create appropriate grants).
- ✗
Create an SCP in account A that allows kms:Decrypt for all accounts, avoiding changes to the key policy.
Why it's wrong here
Service Control Policies (SCPs) are organization-level guardrails. Even if an SCP allows kms:Decrypt, KMS can still deny the call if the CMK’s key policy does not allow it. SCPs also do not replace the CMK key policy requirement for cross-account authorization.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume IAM permissions alone are sufficient for cross-account KMS operations, forgetting that KMS key policies are resource-based and must explicitly grant access to external principals.
Detailed technical explanation
How to think about this question
KMS key policies are resource-based policies that define who can use the key, and for cross-account access, the key policy must explicitly specify the external IAM role ARN as a principal. IAM policies alone are insufficient because the key policy is evaluated first; if it denies or omits the external principal, the request fails regardless of IAM permissions. This is a common pattern in AWS where resource-based policies (like KMS key policies or S3 bucket policies) must explicitly allow cross-account principals, while IAM policies handle permissions within the same account.
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: Update the KMS key policy in account A to allow the account B role principal to use kms:Decrypt on the key. — Option B is correct because cross-account access to a customer managed KMS key (CMK) requires the key policy to explicitly grant the external IAM role principal the necessary permissions (e.g., kms:Decrypt). Even if the IAM role in Account B has an IAM policy allowing kms:Decrypt, the KMS key policy in Account A acts as a resource-based policy that must also allow the action; without this, the request is denied by the key policy, as shown in CloudTrail.
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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Last reviewed: Jun 11, 2026
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