A “third-party watermark” is inserted into a file or files uploaded by a client to a “storing party” such as a file backup server. The third-party watermark may contain information about the upload itself, such as time and date of the upload and the identity of the client. The third-party watermark may also contain authentication information received from the client or elsewhere that establishes that the client is in proper possession of the file, e.g., it is not a “bootlegged” copy.