S3 / Cloudfront Download Restrictions
If you are the owner and you have S3 & EC2 on the same account, then make your files Read/Write only to you. Give permission (Full Control) only to the owner
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To understand Amazon S3 ACL - please read this article: http://docs.amazonwebservices.com/AmazonS3/latest/dev/index.html?ManageACLsUsingConsole.html
Amazon has a guide on how to do this with S3 permissions and CloudFront: