SCS-C02 KMS key policy Practice Question
A company is using AWS KMS to encrypt S3 objects. The security team wants to ensure that only a specific IAM role can decrypt objects in a particular S3 bucket. Which KMS key policy configuration should be used?
⚠ Common exam trap
SCS-C02 often tests the misconception that S3 bucket policies can control KMS decrypt — candidates conflate S3 authorization with KMS authorization, but KMS key policies are the authoritative gate.
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
✓
Add a condition that allows decrypt only when kms:ViaService is s3.amazonaws.com and the caller role matches the specific role ARN.
A KMS key policy with a condition restricting kms:Decrypt to the specific IAM role ARN and kms:ViaService set to s3.amazonaws.com ensures only that role, acting through S3, can decrypt objects. This is the authoritative control because KMS key policies govern all access to the CMK, and the ViaService condition prevents the role from using the key directly outside S3.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Add a condition that allows decrypt only when kms:ViaService is s3.amazonaws.com and the caller role matches the specific role ARN.
Why this is correct
The kms:ViaService condition restricts key usage to requests routed through S3, while the principal or ARN condition limits decryption to the named IAM role. Together they enforce least privilege, ensuring no other principal or direct KMS call can decrypt the bucket's objects.
- ✗
Use an S3 bucket policy that denies decrypt for all principals except the specific IAM user.
Why it's wrong here
Incorrect. S3 bucket policies cannot deny or allow KMS actions. KMS decrypt permissions are controlled by KMS key policies and IAM policies. Additionally, the option mentions an IAM user, not a role.
- ✗
Configure the IAM role with a policy that allows kms:Decrypt for the specific KMS key.
Why it's wrong here
Incorrect. While the IAM role can be granted kms:Decrypt via an IAM policy, the KMS key policy must also allow the role. Moreover, this approach does not restrict the decrypt to S3 only; additional conditions like kms:ViaService are needed.
- ✗
Attach a resource-based policy to the S3 bucket that grants decrypt permission to the IAM role.
Why it's wrong here
Incorrect. S3 bucket policies are resource-based policies for S3 actions, not for KMS actions. They cannot grant KMS decrypt permissions. Only KMS key policies (or IAM policies with key policy allow) can grant KMS decrypt.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.