The answer is that the S3 bucket policy does not allow the role to call s3:GetObject. While the KMS key policy correctly grants decrypt access to the IAM role, decrypting an S3 encrypted object requires two distinct permissions: the KMS decrypt action on the key and the S3 GetObject action on the bucket. Even with perfect KMS permissions, if the S3 bucket policy explicitly denies or omits s3:GetObject for that role, the overall request fails due to the combined effect of resource-based policies. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how cross-service authorization works—specifically that KMS key policies and S3 bucket policies must both allow the necessary actions for decryption to succeed. A common trap is focusing solely on the KMS policy while overlooking the S3 side. Remember the two-step rule: to decrypt an S3 object, you need both kms:Decrypt and s3:GetObject—check both policies.
DEA-C01 Data Security and Governance Practice Question
This DEA-C01 practice question tests your understanding of data security and governance. 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.
Refer to the exhibit. A data engineer creates this KMS key policy. An IAM role in account 123456789012 is granted decrypt access to the key. However, when the DataAnalystRole tries to decrypt an S3 object encrypted with this key, the operation fails. What is the most likely reason?
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.
The S3 bucket policy does not allow the role to call s3:GetObject
Even with decrypt permission, the role needs s3:GetObject permission on the encrypted object.
B
The KMS key is in a different region than the S3 bucket
Why wrong: Cross-region access is possible with appropriate policies, but the key policy is in us-east-1; if bucket is in another region, cross-region calls are allowed if both policies permit.
C
The role does not have permission to call kms:DescribeKey
Why wrong: The policy grants DescribeKey; decryption only requires Decrypt, not DescribeKey.
D
The KMS key policy does not grant kms:Decrypt permission to the role
Why wrong: The policy grants kms:Decrypt and kms:DescribeKey.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The S3 bucket policy does not allow the role to call s3:GetObject
KMS key policies grant access to principals. However, if the S3 bucket policy does not allow the role to call kms:Decrypt, the combination of policies might still deny. But the key policy itself grants decrypt. A common issue is that the S3 bucket policy might not allow the s3:GetObject action, or the role might not have S3 permissions. Another possibility is that the KMS key is in a different region (us-east-1) but the S3 object is in another region, causing cross-region access which is not allowed by default. However, the most likely reason based on typical exam scenarios is that the S3 bucket policy does not grant the necessary S3 permissions.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 S3 bucket policy does not allow the role to call s3:GetObject
Why this is correct
Even with decrypt permission, the role needs s3:GetObject permission on the encrypted object.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The KMS key is in a different region than the S3 bucket
Why it's wrong here
Cross-region access is possible with appropriate policies, but the key policy is in us-east-1; if bucket is in another region, cross-region calls are allowed if both policies permit.
✗
The role does not have permission to call kms:DescribeKey
Why it's wrong here
The policy grants DescribeKey; decryption only requires Decrypt, not DescribeKey.
✗
The KMS key policy does not grant kms:Decrypt permission to the role
Why it's wrong here
The policy grants kms:Decrypt and kms:DescribeKey.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DEA-C01 NAT questions on configuration and troubleshooting.
Data Security and Governance — This question tests Data Security and Governance — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The S3 bucket policy does not allow the role to call s3:GetObject — KMS key policies grant access to principals. However, if the S3 bucket policy does not allow the role to call kms:Decrypt, the combination of policies might still deny. But the key policy itself grants decrypt. A common issue is that the S3 bucket policy might not allow the s3:GetObject action, or the role might not have S3 permissions. Another possibility is that the KMS key is in a different region (us-east-1) but the S3 object is in another region, causing cross-region access which is not allowed by default. However, the most likely reason based on typical exam scenarios is that the S3 bucket policy does not grant the necessary S3 permissions.
What should I do if I get this DEA-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DEA-C01 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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