A decoder for descrambling encoded satellite transmissions comprises an internal security element and a replaceable security module. The program signal is scrambled with a key and then the key itself is twice-encrypted and multiplexed with the scrambled program signal. The key is first encrypted with...http://www.google.com.au/patents/US5029207?utm_source=gb-gplus-sharePatent US5029207 - External security module for a television signal decoder
