Practically secure update of scrambling scheme

Content distributed by broadcast and multicast services is often encrypted (scrambled) to protect copyrighted material. When any cryptanalysis of the current cryptographic scheme (scrambling scheme) used in such services is found, the scheme must be updated. However, the scheme cannot be updated suddenly because a lot of subscribers have receivers with the current scheme. We propose two cryptographic scheme updating methods. They have trade-off relationship between security and transmission bit rate. They use both current and new cryptographic schemes simultaneously, but their transmission bit rates do not need to be doubled. In addition, they are practically secure from the viewpoint of service quality.