| 摘要 |
<p>A method for communication, comprising distributing over a communication network multiple channels of digital content, each channel being encrypted using different, channel-specific control words (CW) (i.e. keys), such that each of the channel-specific control words is derivable from the other channel-specific control words by authorised receivers of the channels on the communication network. The method further comprises transmitting over the channels of the communication network different, channel-specific entitlement control messages (ECMs) from which the control words are derivable. The method may involve the distribution of a master control word (MCW), from which any of the channel-specific control words may be derived. Also described are communication apparatuses operating in accordance with the method, and a security module for use in a receiver configured to receive and decrypt multiple channels of digital content. Embodiments describe application of the described techniques to a conditional access system for selective digital video distribution (i.e. video on demand â VOD). By ensuring that channel-specific control words are derivable from other channel-specific control word, the duration for channel changes may be reduced as it is not necessary to await subsequent ECMs to provide control words for user-requested channels.</p> |
