Secure De-duplications

Files are encrypted on the client side before being uploaded to the cloud, and will be decrypted on the client side after being downloaded to local. The encryption key is kept by the clients. The encryption keys are acquired by the clients from some remote entity, in a privacy-preserving way that the remote entity is not able to infer or distinguish the file content from the requests from all clients, but this remote entity will ensure that the same file content will derive the same encryption key. Thanks to this feature, files across multiple clients can be de-duplicated. Only one copy of a file with unique content (in its encrypted form) will be stored in the cloud server. When duplicated files are deleted, only the links of the ownership will be removed. The file copy in the cloud will be removed only when the file is unique across all clients.