Question 1,420 of 247
How to Enforce Specific KMS Key for S3 Encryption Using Bucket Policy
A company stores sensitive data in an S3 bucket. The security team requires that all objects uploaded to the bucket be encrypted at rest using an AWS KMS customer-managed key. Which S3 bucket policy statement should be added to enforce this requirement?
Quick Answer
The correct answer is the bucket policy that uses a Deny effect with conditions checking both `s3:x-amz-server-side-encryption` equals `aws:kms` and `s3:x-amz-server-side-encryption-aws-kms-key-id` is not null. This works because the first condition ensures the object is encrypted with KMS, while the second condition enforces that a specific KMS key ID is provided, preventing the use of the default AWS-managed KMS key or any other customer key. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of how to combine StringNotEquals and Null conditions to enforce a specific KMS key for S3 encryption, a common security requirement for sensitive data. A frequent trap is choosing a policy that only requires SSE-KMS without restricting the key ID, which would allow any KMS key. Remember the mnemonic "Deny the default, demand the specific key" to recall that you must explicitly deny uploads missing both the `aws:kms` value and a non-null key ID.
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
✓
{"Effect":"Deny","Principal":"*","Action":"s3:PutObject","Resource":"arn:aws:s3:::bucket/*","Condition":{"StringNotEquals":{"s3:x-amz-server-side-encryption":"aws:kms"},"Null":{"s3:x-amz-server-side-encryption-aws-kms-key-id":"true"}}}
It denies any PutObject request that does not use KMS encryption (s3:x-amz-server-side-encryption != 'aws:kms') OR that does not include a KMS key ID (s3:x-amz-server-side-encryption-aws-kms-key-id is null). This enforces that objects are encrypted with a KMS customer-managed key. Option A is incorrect because it denies only if encryption is not KMS, but allows KMS without requiring a key ID, which could permit using the default AWS-managed KMS key rather than a customer-managed key. Option B is incorrect because it denies only if no encryption header is present, but allows any encryption type including SSE-S3 or SSE-KMS without a key ID. Option D is incorrect because it denies encryption that is not AES256, which is SSE-S3, and would incorrectly allow SSE-KMS.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
{"Effect":"Deny","Principal":"*","Action":"s3:PutObject","Resource":"arn:aws:s3:::bucket/*","Condition":{"StringNotEquals":{"s3:x-amz-server-side-encryption":"aws:kms"}}}
Why it's wrong here
This denies uploads that are not using SSE-KMS, but does not require a specific KMS key, so users could use any KMS key.
- ✗
{"Effect":"Deny","Principal":"*","Action":"s3:PutObject","Resource":"arn:aws:s3:::bucket/*","Condition":{"Null":{"s3:x-amz-server-side-encryption":"true"}}}
Why it's wrong here
This only denies uploads without any encryption header, but allows any type of encryption including SSE-S3.
- ✓
{"Effect":"Deny","Principal":"*","Action":"s3:PutObject","Resource":"arn:aws:s3:::bucket/*","Condition":{"StringNotEquals":{"s3:x-amz-server-side-encryption":"aws:kms"},"Null":{"s3:x-amz-server-side-encryption-aws-kms-key-id":"true"}}}
Why this is correct
This denies uploads that do not use SSE-KMS and also ensures the KMS key ID is present (though not specific key). However, to enforce a specific key, a condition on the key ID is needed. This statement is a common baseline.
- ✗
{"Effect":"Deny","Principal":"*","Action":"s3:PutObject","Resource":"arn:aws:s3:::bucket/*","Condition":{"StringNotEquals":{"s3:x-amz-server-side-encryption":"AES256"}}}
Why it's wrong here
This denies uploads that do not use SSE-S3, which is not the required KMS 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 |
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Same concept, more angles
2 more ways this is tested on SOA-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 company has an S3 bucket that stores sensitive customer data. The security team requires that all objects in the bucket be encrypted at rest using AWS KMS. An administrator notices that some objects are not encrypted. What is the MOST efficient way to enforce encryption for future uploads?
hard- A.Use an SCP to require KMS encryption for all S3 actions.
- B.Use AWS Config to detect unencrypted objects and automatically encrypt them.
- ✓ C.Add a bucket policy that denies s3:PutObject unless the request includes the x-amz-server-side-encryption header set to aws:kms.
- D.Enable S3 default encryption on the bucket with KMS.
Why C: Adding a bucket policy that denies s3:PutObject unless the request includes the x-amz-server-side-encryption header set to aws:kms is the most efficient way to enforce encryption for future uploads. This policy prevents any upload without proper encryption. Option A is incorrect because SCPs (Service Control Policies) limit IAM permissions but do not directly enforce encryption on S3 objects. Option B is incorrect because AWS Config can detect unencrypted objects but does not automatically encrypt them; it is a reactive measure. Option D is incorrect because S3 default encryption can be overridden by the client specifying a different encryption header in the upload request.
Variation 2. A company has an S3 bucket that stores sensitive customer data. The security team requires that all objects uploaded to the bucket must be encrypted at rest using AWS KMS with a specific customer managed key. Which bucket policy condition should be used to enforce this?
hard- A."Condition": {"StringEquals": {"s3:x-amz-server-side-encryption": "aws:kms"}}
- ✓ B."Condition": {"StringEquals": {"s3:x-amz-server-side-encryption": "aws:kms", "s3:x-amz-server-side-encryption-aws-kms-key-id": "arn:aws:kms:us-east-1:123456789012:key/abc123"}}
- C."Condition": {"StringEquals": {"s3:x-amz-server-side-encryption-aws-kms-key-id": "arn:aws:kms:us-east-1:123456789012:key/abc123"}}
- D."Condition": {"Null": {"s3:x-amz-server-side-encryption": "false"}}
Why B: It uses both conditions: 's3:x-amz-server-side-encryption' set to 'aws:kms' ensures that objects are encrypted with SSE-KMS, and 's3:x-amz-server-side-encryption-aws-kms-key-id' set to the specific key ARN ensures that only the designated customer managed key is used. Option A enforces KMS encryption but does not restrict which KMS key, allowing any managed key. Option C enforces a specific key ARN but does not require the encryption header to be present, which could allow objects without encryption if the key ID header is omitted (though in practice, the key ID is only valid with SSE-KMS, the condition alone is not sufficient to guarantee encryption). Option D uses a 'Null' condition incorrectly and would not properly enforce encryption.
Last reviewed: Jun 20, 2026
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