Difference in Functionality and Cost When Enabling S3 Bucket Key for Default Encryption

Enabling the S3 Bucket Key feature for default encryption in your S3 bucket can have significant impacts on both functionality and cost:

1. Functional Difference:

When you enable the S3 Bucket Key, Amazon S3 uses a bucket-level key that is generated by AWS Key Management Service (KMS) instead of using individual KMS keys for each object. This bucket-level key is then used to create unique data keys for objects within the bucket.

The main functional differences are:

• Reduced API calls to KMS: S3 will make fewer calls to KMS to complete encryption operations, as it uses the bucket-level key to generate object-level keys.
• Encryption consistency: All new objects added to the bucket will use the S3 Bucket Key for encryption, ensuring a consistent approach across the bucket.
• Granular control: You can still override the bucket-level setting for specific objects if needed, allowing for object-level control when required.

It's important to note that enabling the S3 Bucket Key doesn't change the encryption algorithm or the security of your data. It's primarily an optimization of how the encryption keys are managed and used.

2. Cost Impact:

Enabling the S3 Bucket Key can lead to significant cost savings, especially for workloads that involve a large number of objects or frequent access to encrypted data:

• Reduced KMS request costs: By decreasing the number of API calls made to KMS, you can reduce the associated costs by up to 99%.
• Savings on read-heavy workloads: The recent improvements to S3 Bucket Keys have enhanced savings for read-heavy workloads, particularly beneficial for customers using temporary credentials or data analytics services.
• No additional charges: There are no extra charges for using S3 Bucket Keys. The feature itself is available at no additional cost.

The cost savings can be substantial. For example, some AWS customers have reported savings of 50% to 95% on their KMS request costs after implementing S3 Bucket Keys.

It's worth noting that the cost benefits are most noticeable for buckets with a high volume of objects or frequent access patterns. For buckets with low object counts or infrequent access, the cost difference may be less significant.

In conclusion, enabling the S3 Bucket Key can provide substantial cost savings without compromising security, making it a recommended best practice for many use cases, especially those involving compliance requirements for data encryption or large-scale data storage and access patterns.
Sources
Reducing the cost of SSE-KMS with Amazon S3 Bucket Keys - Amazon Simple Storage Service
Amazon S3 Bucket Keys reduce the costs of Server-Side Encryption with AWS Key Management Service (SSE-KMS)
Enhance savings for read-heavy workloads with Amazon S3 Bucket Keys | AWS Storage Blog

The bucket key option is only relevant for SSE-KMS encryption. It doesn't have any effect with SSE-S3.

It's good to leave the option enabled, despite using SSE-S3, simply so that if you decide to switch to SSE-KMS later or if individual objects are uploaded with SSE-KMS selected at the object level, the SSE-KMS encryption process will leverage bucket keys for optimising costs.