No, having worked on crypto I feel confident saying that the smart people, especially those who do understand crypto, know, and state, that it cannot be done.
All solution fundamentally boil down to someone has the keys required to decrypt all your content. This is ignoring the other technical costs (now your service has to record all ephemeral keys as well).
It doesn’t matter /who/ has that decryption key, the requirement is that access to the key is guarded only by policy.
You also resort to “with a warrant” but a NSL is effectively a warrant, and I’m fairly sure the DPRC has “warrants”. Who gets to request those keys? The position of the us government is that they have the right to access data from people in other countries, so can they provide a warrant to access that data? Can Germany ask for the data for someone in the US? What if someone travelled through another country - can that country now access those keys?
Note these all questions regarding /legitimate/ access, I’m completely ignoring police officers looking up people they stopped and stealing nudes, of people stealing the information and using that information to steal money, stalking, etc
I am sick and tired of people who say “modern cryptography is amazing, so it must be able to fulfill my unicorn dreams”. Either stop claiming nonsense or provide a prove that it is possible by designing a system that does what you say must be possible.