Ask HN: Storing encrypted, copyrighted material for users

Suppose a web service offers users to store their media data, some of which may be copyrighted. The service would keep metadata separate from the content itself, and that content would be transparently encrypted and decrypted on the user's device, with the keys never leaving that device. Thus, the web service has no way of making any sense of the user's files, but can use the metadata to allow sorting, organising and searching for content.

Would that service be in violation of common copyright terms? Not expecting legal counselling here, just an opinion on whether this is a viable option.