Honestly why not just make a database like GIFCT [0], for classified documents? You could make it possible to both preserve the current status quo (e.g. most social medias already run a check against some databases to prevent you from uploading terrorist content and CSAM) and stem the flow of classified documents. No need to ban E2E encryption, no need to have a watchful eye over everyone on the internet, no need for back doors to scan data on private devices or monitor private chat rooms.

However, somehow I doubt that we want to make a database like that given that, even if the hashes are irreversible, the database itself could be used to glean information.

Of course, a better solution could still be the one suggested in the article: stop over-classifying, and reduce access to classified materials overall.

[0]: https://gifct.org/hsdb/