SSE-KMS: Amazon S3-KMS Managed Encryption Keys
Amazon offers a pay-per-use key management service, AWS KMS. This service can be used to encrypt data on S3 using keys which can be centrally managed and assigned to specific roles and IAM accounts.
The AWS KMS can be used by S3 to encrypt uploaded data. When uploading data encrypted with SSE-KMS, the named key that was used to encrypt the data is retrieved from the KMS service, and used to encode the per-object secret which encrypts the uploaded data. To decode the data, the same key must be retrieved from KMS and used to unencrypt the per-object secret key, which is then used to decode the actual file.
KMS keys can be managed by an organization's administrators in AWS, including having access permissions assigned and removed from specific users, groups, and IAM roles. Only those "principals" with granted rights to a key may access it, hence only they may encrypt data with the key, and decrypt data encrypted with it. This allows KMS to be used to provide a cryptographically secure access control mechanism for data stores on S3.