| 摘要 |
A system for providing conditional access to packetized picture (video), aud io or other data. The system selectively encrypts packetized data (105) such th at transport packets (130, 150) that include header data (131, 151) are unencrypted, while all other transport packets (140, 141) that do not includ e header data are encrypted. This allows the transport packets with the header data (130, 150) to be randomly accessed from a memory (230, 310), which is particularly advantageous for performing "trick modes", such as fast forward and fast reverse, e.g., in a video-on-demand service. After the transport packets are selectively encrypted and stored, transport scrambling and contr ol bits (132, 142, 152) for each packet (130, 140, 150) can be accessed to determine whether the packet is encrypted, and consequently, whether the packet includes header data. If a packet includes header data, it is suitabl e for use in a trick mode since it provides data from the start of a video, audio or other data packet.
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