Proof of purchase

This is another popped-into-my-head, maybe-I've-seen-it-somewhere, don't-remember-where, someone's-probably-thought-of-it things.

If I have some digital content in my store (or on my disk, for that matter), it might be good if my particular copy had a nonce in it and the seller included a digitally signed receipt to the effect that "On this date this person bought this copy of this content for this amount from this seller".

Legitimate copies (backups, versions converted to other formats, etc.) would carry either the original proof of purchase (for verbatim copies) or a link to the original or at least the previous link in the chain. In effect, everything can have a provenance.

This doesn't keep me from stripping out the bits and giving an unsigned, de-nonced version to anyone and everyone for free. Airtight copy protection is an inherently hard if not impossible problem. What it does do, though, is cover me by saying hey, at least I paid for my copy.

Whether it's illegal to possess a copy without accompanying receipt is a separate issue. But chances are pirating in general will continue to be illegal, and having a convincing means of proving non-piracy ought to be worth something.

Conversely, having such a scheme in place makes content without clear provenance inherently suspect. Whether The Man can find out you have such, and if so, prove that you have it on purpose, is another separate issue.