The answer is to add an inline policy to the Lambda role that allows kms:GenerateDataKey and kms:Encrypt actions for the KMS key. This is required because, even when S3 default encryption is enabled with a customer managed KMS key, the Lambda function’s IAM role must explicitly grant the KMS permissions needed to generate a data key and encrypt objects during the PutObject call; the key policy alone only authorizes the role to use the key, but the role’s own permissions policy must also include these actions. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of the dual-layer permission model for KMS—where both the key policy and the IAM role policy must align—and it’s a common trap to assume that S3 default encryption automatically grants the writer the necessary KMS actions. Remember the memory tip: “Key policy says ‘yes you can,’ but the role policy must say ‘do it’”—always verify both sides for Lambda KMS permissions for S3 encryption.
DOP-C02 Security and Compliance Practice Question
This DOP-C02 practice question tests your understanding of security and compliance. 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 company uses AWS Lambda functions to process sensitive data from an SQS queue. The Lambda function writes results to an S3 bucket. The security team requires that all data at rest in S3 be encrypted with a customer managed KMS key, and that the Lambda function only have access to decrypt the queue messages and encrypt the S3 objects. An IAM role is attached to the Lambda function. The engineer has configured the KMS key policy to allow the Lambda role to use the key. However, the Lambda function fails to write to S3 with a 'KMS access denied' error. The engineer verified that the S3 bucket has default encryption enabled with the same KMS key. Which additional step is most likely required?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add an inline policy to the Lambda role that allows kms:GenerateDataKey and kms:Encrypt actions for the KMS key.
Even if the S3 bucket has default encryption, the Lambda function needs kms:GenerateDataKey permission (or kms:Encrypt) in its IAM role to encrypt objects when writing. The key policy allows the role, but the role's permissions policy must also include the necessary KMS actions. The error indicates that the role lacks the appropriate KMS action.
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.
✗
Disable default encryption on the S3 bucket and configure the Lambda to use SSE-S3.
Why it's wrong here
Security team requires customer managed KMS key, so SSE-S3 is not acceptable.
✓
Add an inline policy to the Lambda role that allows kms:GenerateDataKey and kms:Encrypt actions for the KMS key.
Why this is correct
The role needs permissions to use the KMS key for encrypting S3 objects.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Grant the Lambda role s3:PutObject permission on the bucket.
Why it's wrong here
S3 permissions are not the issue; the error is KMS access denied.
✗
Modify the KMS key policy to allow the Lambda role to use the key without any conditions.
Why it's wrong here
Key policy likely already allows; the role's permissions are missing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Security and Compliance — This question tests Security and Compliance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Add an inline policy to the Lambda role that allows kms:GenerateDataKey and kms:Encrypt actions for the KMS key. — Even if the S3 bucket has default encryption, the Lambda function needs kms:GenerateDataKey permission (or kms:Encrypt) in its IAM role to encrypt objects when writing. The key policy allows the role, but the role's permissions policy must also include the necessary KMS actions. The error indicates that the role lacks the appropriate KMS action.
What should I do if I get this DOP-C02 question wrong?
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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