This can also be done anonymously for private files, as a feature of backup systems. That way, you could get the ability to prove to anyone that a certain piece

This can also be done anonymously for private files, as a feature of backup systems. That way, you could get the ability to prove to anyone that a certain piece of content was created before that timestamp. It would cost a minimal storage overhead, and the cost of time-stamping can actually decrease with more users.