As to your scheme - circular encryption probably works fine in practice, but there's no theoretical guarantee.
The server cannot read files, but can determine who has uploaded any one cipher- or plaintext. ("Encrypted or unencrypted file".) I think that's still bad.
I wonder if it's possible to fix that too -- that is, could the server receive and store encrypted data from a bunch of people without ever knowing who contributed which data?
The server cannot read files, but can determine who has uploaded any one cipher- or plaintext. ("Encrypted or unencrypted file".) I think that's still bad.