That sounds like a very important key. I'm not discounting the technical merits of your proposal, but I'd worry it'd be very hard to secure the infrastructure u

That sounds like a very important key. I'm not discounting the technical merits of your proposal, but I'd worry it'd be very hard to secure the infrastructure used to create, update, and track those keys. (This is the same logic many use for opposing backdooring encryption, since often it boils down to key escrow)