Optimize S3 Encryption Costs with Bucket Keys

SAA-C03 By islever Editorial Last updated

Design cost-optimized storage solutions.

Short answer

B Use an S3 Bucket Key for server-side encryption with AWS KMS keys (SSE-KMS) on the new objects.

Question

A company runs an environment where data is stored in an Amazon S3 bucket. The objects are accessed frequently throughout the day. The company has strict da ta encryption requirements for data that is stored in the S3 bucket. The company currently uses AWS Key Management Service (AWS KMS) for encryption. The company wants to optimize costs associated with encrypting S3 objects without making additional calls to AWS KMS. Which solution will meet these requirements?
  1. Use server-side encryption with Amazon S3 managed keys (SSE-S3).
  2. Use an S3 Bucket Key for server-side encryption with AWS KMS keys (SSE-KMS) on the new objects. Correct Answer
  3. Use client-side encryption with AWS KMS customer managed keys.
  4. Use server-side encryption with customer-provided keys (SSE-C) stored in AWS KMS.

Key Takeaways

The core concept tested is cost optimization for SSE-KMS, and the common trap is confusing this with the free but less flexible SSE-S3 option or ignoring the 'without making additional calls' constraint.

This question addresses how to reduce AWS KMS request costs when using server-side encryption for Amazon S3 objects that are accessed frequently. It establishes that enabling an S3 Bucket Key allows S3 to cache a data key, significantly reducing the number of API calls made to AWS KMS.

Common mistake: Many learners select Option A (SSE-S3) because it is free and does not involve AWS KMS at all. However, the question states the company currently uses AWS KMS and wants to optimize costs associated with THAT specific setup without changing the fundamental encryption service type, making SSE-KMS with a Bucket Key the precise architectural improvement.

Expert Analysis

Why the Answer Is Correct

Option B is correct because S3 Bucket Keys are designed specifically to reduce the cost of SSE-KMS. When enabled, S3 generates a short-lived bucket-level data key from AWS KMS and caches it. Subsequent encryption operations for new objects use this cached key locally within S3, eliminating the need to call AWS KMS for every single object write. This can reduce KMS request costs by up to 99%.

Why the Other Options Are Wrong

Option A (SSE-S3) uses S3-managed keys which are free, but it changes the encryption model entirely away from AWS KMS, whereas the question implies optimizing the current KMS usage. Option C (Client-side encryption) requires managing encryption in the application code, which increases operational complexity and does not leverage S3's native performance optimizations. Option D is incorrect because SSE-C requires the customer to provide the key with each request, leading to even more overhead than standard SSE-KMS, not less.

Community Comment Notes

As user dragossky noted, "Amazon S3 Bucket Keys reduce the cost of Amazon S3 server-side encryption with AWS Key Management Service (AWS KMS) keys". User FlyingHawk confirmed that "Using S3 Bucket Keys allows you to save on AWS KMS request costs by decreasing your requests to AWS KMS". Some users like elmyth and djhtoon argued for A because it is free, but they missed the nuance that the question asks to optimize the existing KMS-based approach rather than replacing it.

Official Reference

https://docs.aws.amazon.com/AmazonS3/latest/userguide/bucket-key.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When you see 'optimize costs' combined with 'SSE-KMS', always look for 'Bucket Key' as the solution. It is the primary mechanism AWS provides to lower the high per-request cost of KMS encryption at scale.

Frequently Asked Questions

Why not choose SSE-S3 (Option A) since it's free?

While SSE-S3 is free, the question specifies the company currently uses AWS KMS and wants to optimize THAT cost structure. S3 Bucket Keys allow you to keep the flexibility of KMS (like custom policies) while drastically cutting the per-request cost.

Does S3 Bucket Key affect security or performance?

It maintains the same security level as SSE-KMS but improves performance and reduces latency because S3 doesn't have to wait for a remote KMS API call for every single object upload.

More SAA-C03 FAQ →

Community Votes

B
73%
A
27%

73% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. 8621a7c 👍 7 Answered B
    reduce cost https://docs.aws.amazon.com/AmazonS3/latest/userguide/bucket-key.html
  2. elmyth 👍 3 Answered A
    All S3 are now encrypted by default and free with SSE-S3, but B is also possible.
  3. dragossky 👍 2 Answered B
    Amazon S3 Bucket Keys reduce the cost of Amazon S3 server-side encryption with AWS Key Management Service (AWS KMS) keys (SSE-KMS). Using a bucket-level key for SSE-KMS can reduce AWS KMS request costs by up to 99 percent by decreasing the request traffic from Amazon S3 to AWS KMS. With a few clicks in the AWS Management Console, and without any changes to your client applications, you can configure your bucket to use an S3 Bucket Key for SSE-KMS encryption on new objects.
  4. djhtoon 👍 2
    optimize costs associated with encrypting S3 objects without making additional calls to AWS KMS. The answer should be A and it also optimize costs- free.
  5. FlyingHawk 👍 1 Answered B
    When you configure your bucket to use an S3 Bucket Key for SSE-KMS, AWS generates a short-lived bucket-level key from AWS KMS, then temporarily keeps it in S3. This bucket-level key will create data keys for new objects during its lifecycle. S3 Bucket Keys are used for a limited time period within Amazon S3, reducing the need for S3 to make requests to AWS KMS to complete encryption operations. This reduces traffic from S3 to AWS KMS, allowing you to access AWS KMS-encrypted objects in Amazon S3 at a fraction of the previous cost.
Show 2 more comments
  1. FlyingHawk 👍 1 Answered B
    Using S3 Bucket Keys allows you to save on AWS KMS request costs by decreasing your requests to AWS KMS for Encrypt, GenerateDataKey, and Decrypt operations through the use of a bucket-level key. By design, subsequent requests that take advantage of this bucket-level key do not result in AWS KMS API requests or validate access against the AWS KMS key policy.
  2. yangbo 👍 1 Answered A
    without making additional calls to AWS KMS

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