Interpreting S3 Bucket Policy That Requires SSE-KMS

Exhibit

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Principal": "*",
      "Action": "s3:PutObject",
      "Resource": "arn:aws:s3:::example-bucket/*",
      "Condition": {
        "StringNotEquals": {
          "s3:x-amz-server-side-encryption": "aws:kms"
        }
      }
    }
  ]
}

Refer to the exhibit. A data engineer applies the following S3 bucket policy to an S3 bucket. What does this policy enforce?

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

✓

Requires that all objects uploaded to the bucket be encrypted with SSE-KMS

The bucket policy uses a Deny effect with a condition that checks if the s3:x-amz-server-side-encryption header is not 'aws:kms'. This means any PutObject request that does not use SSE-KMS will be denied. Therefore, the policy enforces that all objects uploaded must be encrypted with SSE-KMS. Option A is incorrect because the policy does not mention SSE-S3; it denies if not SSE-KMS. Option B is incorrect because it requires SSE-KMS, not SSE-S3. Option C is incorrect because the policy only allows SSE-KMS, not any type of server-side encryption. Option D is correct.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Denies all uploads unless SSE-S3 is used
Why it's wrong here
The policy permits uploads using the named algorithm rather than denying everything else; a Deny effect with a NotStringEquals condition would be needed to block other encryption types. It is tempting because Deny statements commonly enforce encryption, but this policy's effect is Allow.
✗
Allows only SSE-S3 encrypted uploads
Why it's wrong here
The policy allows the named algorithm but does not exclude other encryption methods, so uploads using SSE-KMS or SSE-C still succeed. It is tempting because the condition names one algorithm, yet Allow with a matching condition does not restrict alternatives.
✗
Allows any type of server-side encryption
Why it's wrong here
The policy's condition keys restrict uploads to a specific encryption algorithm, so it does not permit every server-side encryption type. It is tempting because bucket policies can allow broad encryption, but the explicit StringEquals on the encryption header narrows the permitted set.
✓
Requires that all objects uploaded to the bucket be encrypted with SSE-KMS
Why this is correct
The policy's Deny on s3:PutObject triggers when the s3:x-amz-server-side-encryption header is absent or does not equal aws:kms, so uploads must specify SSE-KMS encryption. This enforces the SSE-KMS requirement rather than merely AES256 or transport encryption.

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

Go deeper

Interpreting S3 Bucket Policy That Requires SSE-KMS

Exhibit

Why each option matters

Option-by-option breakdown

AWS S3 Storage Class Comparison

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