MP3 and PDF Digital Downloads with Amazon S3

I am selling digital MP3’s and PDF’s on a site using Woocommerce. The files are hosted on Amazon S3.

In Woocommerce settings for the individual product, I insert the amazonaws URL for the hosted file. But when you try to access that file to download from Amazon S3, you don’t have access since its ACL is private. I can open the ACL for the file to public but then anybody that has that file link can share the file. I know that Woocommerce sends an encrypted link to the file and I can limit how many times they download through that link. But what if someone gets the name of the folder/file in the Amazon bucket and can access directly (since the ACL is public)?

I have read these posts that don’t offer an answer for similar questions:

http://wordpress.org/support/topic/download-problem-with-amazon-s3?replies=4
http://wordpress.org/support/topic/digital-downloads-with-amazon-s3?replies=3

I could use a script to sign the URL, but then how would I get that URL into the Woocommerce product download box?

http://wordpress.org/plugins/woocommerce/