Digital information protecting method and system

This invention provides a digital information protecting method executed in an author and a client computer, both having a predetermined information processing software to process a piece of digital information. In the author computer, receive a content key from a server and encrypt the piece of digital information by the content key. Encrypt the content key by a predetermined key encrypting process. Transmit the encrypted information and encrypted content key to the client computer. In the client computer, decrypt the encrypted content key by a corresponding predetermined decrypting process. Decrypt the encrypted information by the content key so that the piece of digital information can be used by the client computer. No matter the client computer is on-line or off-line, it can get the key and decrypt the piece of digital information.