The answer is that the KMS key policy does not grant the Lambda role decrypt permission. This is correct because AWS KMS requires a dual-permission model: the IAM policy attached to the Lambda execution role must allow kms:Decrypt, but the KMS key policy itself must also explicitly grant that same permission to the role as a principal. Since key policies are resource-based, they act as the final gatekeeper—without that explicit grant, the request is denied even if the IAM policy is perfectly configured. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how KMS key policies override IAM permissions for cross-account or cross-service access, a common trap where candidates assume IAM alone is sufficient. A reliable memory tip is “IAM says yes, but the key says no”—always check both policies when troubleshooting Lambda KMS decrypt access denied errors.
DEA-C01 Data Store Management Practice Question
This DEA-C01 practice question tests your understanding of data store management. 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.
A data engineer is troubleshooting an access denied error when an AWS Lambda function tries to decrypt an object encrypted with the KMS key 'abc123'. The Lambda function's execution role has the above policy attached. What is the likely cause of the error?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The KMS key policy does not grant the Lambda role decrypt permission
The error occurs because KMS requires both the IAM policy and the key policy to grant the necessary permissions. While the IAM policy attached to the Lambda execution role includes kms:Decrypt, the KMS key policy for 'abc123' does not explicitly grant the Lambda role permission to call kms:Decrypt. Since KMS key policies act as a resource-based policy, they must allow the principal (the Lambda role) to perform the action; otherwise, the request is denied even if the IAM policy allows it.
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.
✗
The Deny statement blocks all decrypt requests
Why it's wrong here
The Deny only applies when source account is not 123456789012; same account, so not denied.
✗
The Lambda function does not have permission to call kms:GenerateDataKey
Why it's wrong here
The policy allows GenerateDataKey on the key.
✓
The KMS key policy does not grant the Lambda role decrypt permission
Why this is correct
Key policies must also grant access; IAM alone may not be sufficient.
Related concept
Read the scenario before looking for a memorised answer.
✗
The IAM policy does not include kms:Decrypt permission
Why it's wrong here
The policy includes kms:Decrypt on the specific key.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume IAM permissions alone are sufficient for KMS operations, overlooking that KMS key policies must explicitly grant access to the IAM role, which is a common source of access denied errors in cross-account or cross-service scenarios.
Detailed technical explanation
How to think about this question
KMS uses a dual authorization model: an IAM principal must have both an IAM policy allowing the action and a key policy (or a grant) allowing the same action on the specific key. Key policies are resource-based policies attached directly to the CMK, and if they do not include the Lambda execution role as a principal with kms:Decrypt, the request fails with an access denied error, even if the IAM policy is correctly configured. In practice, this often occurs when a key is created with a default key policy that only allows the root account, and the data engineer forgets to add the Lambda role as a key user.
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.
Data Store Management — This question tests Data Store Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The KMS key policy does not grant the Lambda role decrypt permission — The error occurs because KMS requires both the IAM policy and the key policy to grant the necessary permissions. While the IAM policy attached to the Lambda execution role includes kms:Decrypt, the KMS key policy for 'abc123' does not explicitly grant the Lambda role permission to call kms:Decrypt. Since KMS key policies act as a resource-based policy, they must allow the principal (the Lambda role) to perform the action; otherwise, the request is denied even if the IAM policy allows it.
What should I do if I get this DEA-C01 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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