Of course Dropbox knows the keys. If they didn’t, you wouldn’t be able to access your files on so many platforms (web, desktop, iPhone) and you wouldn’t be able to easily share folders with others.
Even though it isn’t spelled out, I’ve always suspected that many actual backup services such as Backblaze don’t know the key if I decide to encrypt my backups.
I don't think the "many clients" argument holds. If done properly, Dropbox would (deterministically) generate the keys from your username and password on the client every time you log in and encrypt/decrypt stuff there.
You're right that sharing folders between different accounts would require having a key shared between the clients and thus stored on the Dropbox servers as well.
However, they could generate the key for the shared folder, give it to you and your buddies and yet store it encrypted using your master key generated from your username and password. Then it would be accessible to you, but Drobox would not be able to decrypt it without getting your password.
It looks like they're not doing that, but hypothetically, they could. And the examples you cite would not be impossible to deal with.
Of course, this would require much more work, would be tricky to get right and they'd have Thomas Ptacek on their back for using JavaScript crypto in the browser.
You are absolutely right in terms of what is technically possible. My point was more that it’s not really realistic to use crypto like that with a Dropbox-like service.
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Of course Dropbox knows the keys. If they didn’t, you wouldn’t be able to access your files on so many platforms (web, desktop, iPhone) and you wouldn’t be able to easily share folders with others.
Even though it isn’t spelled out, I’ve always suspected that many actual backup services such as Backblaze don’t know the key if I decide to encrypt my backups.
I don't think the "many clients" argument holds. If done properly, Dropbox would (deterministically) generate the keys from your username and password on the client every time you log in and encrypt/decrypt stuff there.
You're right that sharing folders between different accounts would require having a key shared between the clients and thus stored on the Dropbox servers as well.
However, they could generate the key for the shared folder, give it to you and your buddies and yet store it encrypted using your master key generated from your username and password. Then it would be accessible to you, but Drobox would not be able to decrypt it without getting your password.
It looks like they're not doing that, but hypothetically, they could. And the examples you cite would not be impossible to deal with.
Of course, this would require much more work, would be tricky to get right and they'd have Thomas Ptacek on their back for using JavaScript crypto in the browser.
You are absolutely right in terms of what is technically possible. My point was more that it’s not really realistic to use crypto like that with a Dropbox-like service.