Using server-side encryption with AWS KMS keys (SSE-KMS)

Server-side encryption is the encryption of data at its destination by the application or service that receives it.

Amazon S3 automatically enables server-side encryption with Amazon S3 managed keys (SSE-S3) for new object uploads.

Unless you specify otherwise, buckets use SSE-S3 by default to encrypt objects. However, you can choose to configure buckets to use server-side encryption with AWS Key Management Service (AWS KMS) keys (SSE-KMS) instead. For more information, see Specifying server-side encryption with AWS KMS (SSE-KMS).

AWS KMS is a service that combines secure, highly available hardware and software to provide a key management system scaled for the cloud. Amazon S3 uses server-side encryption with AWS KMS (SSE-KMS) to encrypt your S3 object data. Also, when SSE-KMS is requested for the object, the S3 checksum (as part of the object's metadata) is stored in encrypted form. For more information about checksum, see Checking object integrity.

If you use KMS keys, you can use AWS KMS through the AWS Management Console or the AWS KMS API to do the following:

The security controls in AWS KMS can help you meet encryption-related compliance requirements. You can use these KMS keys to protect your data in Amazon S3 buckets. When you use SSE-KMS encryption with an S3 bucket, the AWS KMS keys must be in the same Region as the bucket.

There are additional charges for using AWS KMS keys. For more information, see AWS KMS key concepts in the AWS Key Management Service Developer Guide and AWS KMS pricing.

Permissions

To upload an object encrypted with an AWS KMS key to Amazon S3, you need kms:GenerateDataKey permissions on the key. To download an object encrypted with an AWS KMS key, you need kms:Decrypt permissions. For information about the AWS KMS permissions that are required for multipart uploads, see Multipart upload API and permissions.

AWS KMS keys

When you use server-side encryption with AWS KMS (SSE-KMS), you can use the default AWS managed key, or you can specify a customer managed key that you have already created. AWS KMS supports envelope encryption. S3 uses the AWS KMS features for envelope encryption to further protect your data. Envelope encryption is the practice of encrypting your plaintext data with a data key, and then encrypting that data key with a KMS key. For more information about envelope encryption, see Envelope encryption in the AWS Key Management Service Developer Guide.

If you don't specify a customer managed key, Amazon S3 automatically creates an AWS managed key in your AWS account the first time that you add an object encrypted with SSE-KMS to a bucket. By default, Amazon S3 uses this KMS key for SSE-KMS.

If you want to use a customer managed key for SSE-KMS, create a symmetric encryption customer managed key before you configure SSE-KMS. Then, when you configure SSE-KMS for your bucket, specify the existing customer managed key. For more information about symmetric encryption key, see Symmetric encryption KMS keys in the AWS Key Management Service Developer Guide.

Creating a customer managed key gives you more flexibility and control. For example, you can create, rotate, and disable customer managed keys. You can also define access controls and audit the customer managed key that you use to protect your data. For more information about customer managed and AWS managed keys, see Customer keys and AWS keys in the AWS Key Management Service Developer Guide.

Using SSE-KMS encryption for cross-account operations

When using encryption for cross-account operations, be aware of the following:

For more information about when to use customer managed keys and AWS managed KMS keys, see Should I use an AWS managed key or a customer managed key to encrypt my objects in Amazon S3?

SSE-KMS encryption workflow

If you choose to encrypt your data using an AWS managed key or a customer managed key, AWS KMS and Amazon S3 perform the following envelope encryption actions:

When you request that your data be decrypted, Amazon S3 and AWS KMS perform the following actions:

Auditing SSE-KMS encryption

To identify requests that specify SSE-KMS, you can use the All SSE-KMS requests and % all SSE-KMS requests metrics in Amazon S3 Storage Lens metrics. S3 Storage Lens is a cloud-storage analytics feature that you can use to gain organization-wide visibility into object-storage usage and activity. You can also use the SSE-KMS enabled bucket count and % SSE-KMS enabled buckets to understand the count of buckets that (SSE-KMS) for default bucket encryption. For more information, see Assessing your storage activity and usage with S3 Storage Lens. For a complete list of metrics, see S3 Storage Lens metrics glossary.

To audit the usage of your AWS KMS keys for your SSE-KMS encrypted data, you can use AWS CloudTrail logs. You can get insight into your cryptographic operations, such as GenerateDataKey and Decrypt. CloudTrail supports numerous attribute values for filtering your search, including event name, user name, and event source.

Amazon S3 Bucket Keys

When you configure server-side encryption using AWS KMS (SSE-KMS), you can configure your buckets to use S3 Bucket Keys for SSE-KMS. Using a bucket-level key for SSE-KMS can reduce your AWS KMS request costs by up to 99 percent by decreasing the request traffic from Amazon S3 to AWS KMS.

When you configure a bucket to use an S3 Bucket Key for SSE-KMS on new objects, AWS KMS generates a bucket-level key that is used to create unique data keys for objects in the bucket. This S3 Bucket Key is used for a time-limited period within Amazon S3, further reducing the need for Amazon S3 to make requests to AWS KMS to complete encryption operations. For more information about using S3 Bucket Keys, see Reducing the cost of SSE-KMS with Amazon S3 Bucket Keys.

Requiring server-side encryption

To require server-side encryption of all objects in a particular Amazon S3 bucket, you can use a bucket policy. For example, the following bucket policy denies the upload object (s3:PutObject) permission to everyone if the request does not include an x-amz-server-side-encryption-aws-kms-key-id header that requests server-side encryption with SSE-KMS.

{
   "Version":"2012-10-17",
   "Id":"PutObjectPolicy",
   "Statement":[{
         "Sid":"DenyObjectsThatAreNotSSEKMS",
         "Effect":"Deny",
         "Principal":"*",
         "Action":"s3:PutObject",
         "Resource":"arn:aws:s3:::amzn-s3-demo-bucket1/*",
         "Condition":{
            "Null":{
               "s3:x-amz-server-side-encryption-aws-kms-key-id":"true"
            }
         }
      }
   ]
}

To require that a particular AWS KMS key be used to encrypt the objects in a bucket, you can use the s3:x-amz-server-side-encryption-aws-kms-key-id condition key. To specify the KMS key, you must use a key Amazon Resource Name (ARN) that is in the arn:aws:kms:region:acct-id:key/key-id format. AWS Identity and Access Management does not validate if the string for s3:x-amz-server-side-encryption-aws-kms-key-id exists.

For a complete list of Amazon S3 specific condition keys, see Condition keys for Amazon S3 in the Service Authorization Reference.

Encryption context

An encryption context is a set of key-value pairs that contains additional contextual information about the data. The encryption context is not encrypted. When an encryption context is specified for an encryption operation, Amazon S3 must specify the same encryption context for the decryption operation. Otherwise, the decryption fails. AWS KMS uses the encryption context as additional authenticated data (AAD) to support authenticated encryption. For more information about the encryption context, see Encryption context in the AWS Key Management Service Developer Guide.

By default, Amazon S3 uses the object or bucket Amazon Resource Name (ARN) as the encryption context pair:

You can optionally provide an additional encryption context pair by using the x-amz-server-side-encryption-context header in an s3:PutObject request. However, because the encryption context is not encrypted, make sure it does not include sensitive information. Amazon S3 stores this additional key pair alongside the default encryption context. When it processes your PUT request, Amazon S3 appends the default encryption context of aws:s3:arn to the one that you provide.

You can use the encryption context to identify and categorize your cryptographic operations. You can also use the default encryption context ARN value to track relevant requests in AWS CloudTrail by viewing which Amazon S3 ARN was used with which encryption key.

In the requestParameters field of a CloudTrail log file, the encryption context looks similar to the following one.

"encryptionContext": {
    "aws:s3:arn": "arn:aws:s3:::amzn-s3-demo-bucket1/file_name"
}

When you use SSE-KMS with the optional S3 Bucket Keys feature, the encryption context value is the ARN of the bucket.

"encryptionContext": {
    "aws:s3:arn": "arn:aws:s3:::amzn-s3-demo-bucket1"
}

Sending requests for AWS KMS encrypted objects

AWS Signature Version 4 is the process of adding authentication information to AWS requests sent by HTTP. For security, most requests to AWS must be signed with an access key, which consists of an access key ID and secret access key. These two keys are commonly referred to as your security credentials. For more information, see Authenticating Requests (AWS Signature Version 4) and Signature Version 4 signing process.