IMHO a certificate that expires at the same time as the copyright on a film, and DRM that then "fails open", makes the most sense.
You can't build a "time lock" with just encryption primitives. Even if you could build a time lock with just encryption primitives, we don't know when the copyright will expire until the original author has died, since copyright term is life + 70 years.
Someone would have to run a server that specifically chooses to start serving the keys on that date, which is an absurd notion given how absurdly long copyright lasts these days.
If the federal government were interested in passing a law that required this, I'm sure the Library of Congress could run such a server, but no such law exists.
Maybe not through crypto but otherwise perfectly achievable. No reason to achieve this within a file. If you really want to, some fancy solution involving blockchains and smart contracts is probably possible but there is no need.
Require by law all who desire copyright protection to register the work with a governmental agency and place a copy of the work in escrow at an archive as a condition. The archive knows when the copyright expires and starts serving the work to the public from that point forward. This is what to do if a state cares about public domain.
The status quo of corporations abandoning works to bit rot, actual rot, misplacement or fires for decades is strictly worse.
> Require by law all who desire copyright protection to register the work with a governmental agency and place a copy of the work in escrow at an archive as a condition.
You’re just repeating what I already wrote. By all means, call your representatives.
It's the same. "Fail closed" means you can "just" set the system time to be before the expiration date, "fail open" means you can just set the system time to be after.
Either way, having an expiring decryption key is just security theater that harms users.
Spoofing that now gets you in trouble with the FCC as well as the DMCA.
GP’s point stands, you can’t enforce a time lock with cryptography primitives. That’s the fundamental issue with DRM - you’re trying to restrict someone from getting your ciphertext, while at the same time allowing them to get to it if they meet certain (non-cryptographic) conditions, which standard cryptography just can’t do.