On the one-hand I think it would be similar to how usenet works now for binaries. That is, once notified under DMCA (for the USA) and likely similar laws in other western countries you’re duty bound to remove it.

I don’t know if there would be other problems with hosting files when you don’t know what they are. Also in terms of defence against DMCA, how would the original file uploader defend against it when you can’t know what the file is without the key. Person A reports file xyz as infringing their copyright, Uploader B says it doesn’t. Normally you could re-instate it and let the two parties fight it out in court. But, I wonder how it would play out when you hosting the file don’t even know what it is.

I’m really not sure how it would really stack up against copyright law in general and more specifically laws for truly illegal content (e.g. CSAM), since you could be hosting that and never know.

Seems a bit more of a risky venture to me and more a question for an actually qualified legal advisor I’m afraid.