A selective encryption method and apparatus consistent with the invention duplicates selected packets in a file or data stream and multiple encrypts the packets using multiple encryption keys. Each encryption key is valid for a specific segment of time so that changes in entitlement keys used for decryption...http://www.google.com/patents/US20030159140?utm_source=gb-gplus-sharePatent US20030159140 - Selective encryption to enable multiple decryption keys
