Ask HN: How do you protect download links for digital content?

Hello, I am interested in finding out ways of protecting download links for digital content aside the use of adding a Nonce (http://en.wikipedia.org/wiki/Cryptographic_nonce) and a unique URL which expires after first use. I would like to end up with links that when used once, becomes invalid for next use. This is to prevent users from sharing the links with their friends. Links and pointers to resources are appreciated.