There are drawbacks to end-to-end encryption (E2EE). I'm not aware of any E2EE cloud storage systems that have the features Dropbox provides. I would LOVE to know of any that...
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Support at least the big 5 platforms (Android/iOS/Mac/Windows/Linux).
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Have a functional web interface.
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Support sharing and collaboration.
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Have a search feature
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Sync to the local filesystem on a folder-by-folder or even file-by-file basis
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Integrate with other tools (e.g. android file picker)
It's not easy to do all that with E2EE, like a functional web interface, search, and integration.
ProtonMail's search, for example, is limited to subject and metadata, and that's specifically because they DON'T use E2EE for that.
I'm willing to compromise some of this for the sake of E2EE, but I'm not at all surprised that feature-first services are more popular than privacy-first services.
I just checked to see if I missed a big update.
There's still no Linux client, and it cannot sync files on Android (it only supports photo backups).
I can't work around that limitation on Android with FolderSync, either, the way I can with Google Drive, Dropbox, Box, or any WebDAV- or S3-compatible server. Since it uses E2EE, any uploads need to go directly through the app, so integrations are difficult.
It doesn't seem to have a search feature, either, at least not on Android. I can't imagine there's any content-aware search on the web UI, since that can't be done server-side.
There's been some interesting research in homomorphic encryption over the past couple years, which might someday lead to encrypted server-side search. But I think there are still major hurdles to actually implementing it securely and efficiently.