Two-way CA system of digital TV-set and method for ordering and cancelling programm based on it

The invention is used for providing a security authentication between the content provider, network service supplier and users in digital TV system. The method comprises: based on the active authentication of client side, a bidirectional authentication is adopted in order to realize the one-time-pad in the authentication process; said authentication information is encrypted with public key in the smart card of set-top box, and the encrypted authentication information is decrypted with private key in the front end authentication server.