I recently tried to use AWS CLI to upload a folder full of files to S3 using a custom KMS key. This is possible by using the “aws s3api put-object” command, but it’s not possible using the “aws s3 sync” command. If you’re just uploading a few files, this isn’t a big deal, but the frustration grows with each extra file that needs to be uploaded.
The “s3 sync” command is a container for the s3api PUT action, so in order to use it for an entire folder (with a custom KMS key), you would need to write some kind of wrapper for it.
Otherwise, you can use one of the stock encryption keys and upload your entire folder to S3.