specification needed for downloadable expiring links for Amazon S3

I need a specification created for a simple app for creating html pages that make it easy for me to create expiring links for S3 so users who buy my stuff can download files. The app has to make it easy to create these links in a self service fashion, and let my backoffice people re-authorize creation of links for someone who purchased but did not download the link in time. Links should have variable expirys, 24 hours, 48 hours, 72 hours, etc. I am looking for a specification that has an explanation of how to do this, how the app will work, using php/Linux and something that can work within Wordpress or on HTML. If I like your explanation, I will ask you to develop the app.