Reducing the cost of SSE-KMS with Amazon S3 Bucket Keys

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.

S3 Bucket Keys for SSE-KMS

Workloads that access millions or billions of objects encrypted with SSE-KMS can generate large volumes of requests to AWS KMS. When you use SSE-KMS to protect your data without an S3 Bucket Key, Amazon S3 uses an individual AWS KMS data key for every object. In this case, Amazon S3 makes a call to AWS KMS every time a request is made against a KMS-encrypted object. For information about how SSE-KMS works, see Using server-side encryption with AWS KMS keys (SSE-KMS).

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.

Unique bucket-level keys are fetched at least once per requester to ensure that the requester's access to the key is captured in an AWS KMS CloudTrail event. Amazon S3 treats callers as different requesters when they use different roles or accounts, or the same role with different scoping policies. AWS KMS request savings reflect the number of requesters, request patterns, and relative age of the objects requested. For example, a fewer number of requesters, requesting multiple objects in a limited time window, and encrypted with the same bucket-level key, results in greater savings.

When you configure an S3 Bucket Key, objects that are already in the bucket do not use the S3 Bucket Key. To configure an S3 Bucket Key for existing objects, you can use a CopyObject operation. For more information, see Configuring an S3 Bucket Key at the object level.

Amazon S3 will only share an S3 Bucket Key for objects encrypted by the same AWS KMS key. S3 Bucket Keys are compatible with KMS keys created by AWS KMS, imported key material, and key material backed by custom key stores.

Configuring S3 Bucket Keys

You can configure your bucket to use an S3 Bucket Key for SSE-KMS on new objects through the Amazon S3 console, AWS SDKs, AWS CLI, or REST API. With S3 Bucket Keys enabled on your bucket, objects uploaded with a different specified SSE-KMS key will use their own S3 Bucket Keys. Regardless of your S3 Bucket Key setting, you can include the x-amz-server-side-encryption-bucket-key-enabled header with a true or false value in your request, to override the bucket setting.

Before you configure your bucket to use an S3 Bucket Key, review Changes to note before enabling an S3 Bucket Key.

Configuring an S3 Bucket Key using the Amazon S3 console

When you create a new bucket, you can configure your bucket to use an S3 Bucket Key for SSE-KMS on new objects. You can also configure an existing bucket to use an S3 Bucket Key for SSE-KMS on new objects by updating your bucket properties. 

For more information, see Configuring your bucket to use an S3 Bucket Key with SSE-KMS for new objects.

REST API, AWS CLI, and AWS SDK support for S3 Bucket Keys

You can use the REST API, AWS CLI, or AWS SDK to configure your bucket to use an S3 Bucket Key for SSE-KMS on new objects. You can also enable an S3 Bucket Key at the object level.

For more information, see the following: 

The following API operations support S3 Bucket Keys for SSE-KMS:

Working with AWS CloudFormation

In AWS CloudFormation, the AWS::S3::Bucket resource includes an encryption property called BucketKeyEnabled that you can use to enable or disable an S3 Bucket Key.

For more information, see Using AWS CloudFormation.

Changes to note before enabling an S3 Bucket Key

Before you enable an S3 Bucket Key, note the following related changes:

IAM or AWS KMS key policies

If your existing AWS Identity and Access Management (IAM) policies or AWS KMS key policies use your object Amazon Resource Name (ARN) as the encryption context to refine or limit access to your KMS key, these policies won't work with an S3 Bucket Key. S3 Bucket Keys use the bucket ARN as encryption context. Before you enable an S3 Bucket Key, update your IAM policies or AWS KMS key policies to use your bucket ARN as the encryption context.

For more information about the encryption context and S3 Bucket Keys, see Encryption context.

CloudTrail events for AWS KMS

After you enable an S3 Bucket Key, your AWS KMS CloudTrail events log your bucket ARN instead of your object ARN. Additionally, you see fewer KMS CloudTrail events for SSE-KMS objects in your logs. Because key material is time-limited in Amazon S3, fewer requests are made to AWS KMS.

Using an S3 Bucket Key with replication

You can use S3 Bucket Keys with Same-Region Replication (SRR) and Cross-Region Replication (CRR).

When Amazon S3 replicates an encrypted object, it generally preserves the encryption settings of the replica object in the destination bucket. However, if the source object is not encrypted and your destination bucket uses default encryption or an S3 Bucket Key, Amazon S3 encrypts the object with the destination bucket’s configuration.

The following examples illustrate how an S3 Bucket Key works with replication. For more information, see Replicating encrypted objects (SSE-S3, SSE-KMS, DSSE-KMS, SSE-C). 

Working with S3 Bucket Keys

For more information about enabling and working with S3 Bucket Keys, see the following sections: