Question 940 of 318
Enforcing S3 KMS Encryption — Bucket Policy and Default Encryption
A security engineer is implementing a data classification policy for an S3 bucket that contains sensitive customer data. The policy requires that all objects be encrypted at rest using AWS KMS and that any attempt to upload an unencrypted object be denied. Which THREE steps should the engineer take to enforce this policy? (Choose THREE.)
Quick Answer
Enable S3 default encryption with the KMS key completes a defense-in-depth strategy for enforcing encryption at rest, rather than acting alone as a silver-bullet control. A customer managed KMS key gives full control over rotation, access policies, and auditing; a bucket policy that denies PutObject unless the s3:x-amz-server-side-encryption header equals aws:kms rejects any upload that does not specify the right encryption; and default encryption acts as a safety net that automatically applies KMS encryption even if the uploader forgets to set the header, so nothing ever lands in the bucket unencrypted. The distinction worth holding onto is between deny-based and default-based enforcement: the bucket policy stops non-compliant requests outright, while default encryption quietly fixes requests that omit encryption instructions, together covering both the reject-bad-requests and auto-correct-missing-instructions cases. This is also why the other options fail: enabling bucket keys only reduces the number of KMS API calls for objects already being encrypted with KMS, it does nothing to enforce that encryption happens at all, and enforcing SSL/TLS only protects data in transit, a separate requirement from encryption at rest. When a question asks you to enforce a policy rather than simply enable a feature, look for the combination of a preventive control, a policy-level deny, and a default that closes any remaining gap.
⚠ Common exam trap
A common trap is to confuse encryption in transit (SSL/TLS) with encryption at rest. The question explicitly asks about encryption at rest, so options related to SecureTransport are irrelevant.
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
✓
Create a customer managed KMS key.
To enforce encryption at rest using AWS KMS for all objects uploaded to an S3 bucket, the security engineer should: - Create a customer managed KMS key (option B) to have full control over the key, including rotation, access policies, and auditing. - Add a bucket policy that denies PutObject if the `s3:x-amz-server-side-encryption` header is not set to `aws:kms` (option D). This ensures any upload without KMS encryption is denied. - Enable S3 default encryption with the KMS key (option E) to automatically encrypt objects even if the upload request does not include encryption headers. Option A (enabling bucket keys) is wrong because it reduces KMS API calls for already KMS-encrypted objects, but does not enforce encryption. Option C (enforcing SSL) addresses encryption in transit, not at rest, and thus does not meet the policy requirement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable S3 bucket keys to reduce KMS API calls.
Why it's wrong here
Does not enforce encryption policy.
- ✓
Create a customer managed KMS key.
Why this is correct
Provides encryption key for S3.
- ✗
Enable bucket policy to enforce SSL (aws:SecureTransport).
Why it's wrong here
Enforces transport encryption, not at-rest encryption.
- ✓
Add a bucket policy that denies PutObject if s3:x-amz-server-side-encryption is not aws:kms.
Why this is correct
Denies unencrypted uploads.
- ✓
Enable S3 default encryption with the KMS key.
Why this is correct
Ensures all objects are encrypted.
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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Same concept, more angles
1 more way this is tested on SCS-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A security engineer is designing a data encryption strategy for an S3 bucket that contains sensitive information. Which TWO of the following are valid options for enforcing encryption at rest?
medium- A.Use an AWS KMS key with automatic key rotation.
- ✓ B.Enable S3 default encryption on the bucket.
- C.Enable AWS CloudTrail to log all object uploads.
- D.Attach an IAM policy to users to require encryption.
- ✓ E.Use a bucket policy to deny PutObject requests without the x-amz-server-side-encryption header.
Why B: The correct options are B and E. Option B: Enabling S3 default encryption on the bucket automatically encrypts objects at rest using the specified encryption method (SSE-S3 or SSE-KMS). Option E: A bucket policy can deny PutObject requests that do not include the x-amz-server-side-encryption header, ensuring objects are uploaded with encryption enforced. Option A is incorrect because using an AWS KMS key with automatic key rotation is a method of encryption, not an enforcement mechanism; the question asks for ways to enforce encryption at rest. Option C is incorrect because AWS CloudTrail logs API calls, not encryption enforcement. Option D is incorrect because IAM policies control user permissions but cannot directly enforce encryption at the service level; they can deny actions but encryption enforcement is typically done via bucket policies or default encryption.
Last reviewed: Jun 20, 2026
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