S3 client side encryption

You can build your own library that encrypts your objects data on the client side before uploading it to Amazon S3. Or you can use the AWS provided SDK. Currently, only the AWS SDK for Java supports client-side encryption.

S3 SSE without request header

You must include the provide a request header, x-amz-server-side-encryption.  S3 SSE

 encrypts each object with a unique key.  Unfortunately there is no way to enable SSE for a bucket where SSE happens automatically. There is no such bucket policy or bucket setting.  Some third party tools take care of automatically using SSE, such as S3 browser: http://s3browser.com/amazon-s3-server-side-encryption.php. You can enable encryption for already uploaded files, and it can also be configured to automatically apply encryption during uploading.