The correct answer is that the application can decrypt objects only when accessing them through S3. This is because the IAM policy for S3 KMS decrypt restriction uses the `kms:ViaService` condition key set to `s3.amazonaws.com`, which explicitly limits the `kms:Decrypt` permission to requests that originate from the S3 service. Without this condition, the role could decrypt via direct KMS API calls or other services, but here the policy enforces that decryption is only allowed when S3 handles the request on behalf of the application. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of service-specific condition keys in IAM policies, a common trap being that students assume `kms:Decrypt` alone grants blanket access. Remember the mnemonic: “Via S3 only” means S3 is the only door through which decryption can enter.
DEA-C01 Data Store Management Practice Question
This DEA-C01 practice question tests your understanding of data store management. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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. An IAM policy is attached to an IAM role used by an application. The application needs to decrypt objects in an S3 bucket using a customer managed KMS key. What is the effect of this policy?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The application can decrypt objects only when accessing them through S3.
The IAM policy grants the `kms:Decrypt` permission with a `kms:ViaService` condition key set to `s3.amazonaws.com`. This condition restricts the decryption operation to only when the request is made through the S3 service. Therefore, the application can decrypt objects only when accessing them through S3, not via direct KMS API calls or other services.
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 application cannot perform any KMS operations.
Why it's wrong here
Encrypt is allowed; decrypt is allowed via S3.
✗
The application can decrypt objects from any service.
Why it's wrong here
Decrypt is denied if not via S3.
✓
The application can decrypt objects only when accessing them through S3.
Why this is correct
The Deny with condition allows decrypt only via S3 service.
Related concept
Read the scenario before looking for a memorised answer.
✗
The application can encrypt but not decrypt objects.
Why it's wrong here
Encrypt is allowed; decrypt is allowed only via S3.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the `kms:ViaService` condition key to trap candidates who assume that granting `kms:Decrypt` alone allows decryption from any source, ignoring the service-specific restriction.
Detailed technical explanation
How to think about this question
The `kms:ViaService` condition key is a powerful IAM condition that restricts KMS key usage to requests made through a specific AWS service (e.g., S3, Lambda, EC2). Under the hood, AWS services like S3 call KMS on behalf of the user, and the condition ensures the `CallerSource` in the KMS request matches the specified service. In real-world scenarios, this prevents an application from using the same role to decrypt data outside the intended service, such as using the AWS CLI to directly call KMS Decrypt, which would fail.
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 application can decrypt objects only when accessing them through S3. — The IAM policy grants the `kms:Decrypt` permission with a `kms:ViaService` condition key set to `s3.amazonaws.com`. This condition restricts the decryption operation to only when the request is made through the S3 service. Therefore, the application can decrypt objects only when accessing them through S3, not via direct KMS API calls or other services.
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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