DVB protocol describes a scarmbling lagorithm. This algorithm is called CSA, Common Scrambling Algorithm. Every provider uses this algorithm.
In it's simplest way of implementing scrambled transmission, the operators scarmbls the service with a fixed key. Every one who is supposed to receive this service gets the fiexed key and he receives the service.
This method is used for connections from one partner to another. It is not convenient for operator to use it for their broadcasting.
In order to broadcast, they use one encryption system: Irdeto2, Viaccess etc How it works? Firstly, the encryption system generates a random key for CSA. The service is scrambled with the key. Then, the key is encrypted by the proprietary (and also TOP SECRET) algorithm of the encryption system. ECM contains the encrypted key.The CAM/smartcard decrypts the key. Then, the receiver gets the key and descrambls the service.
It is possible for an operator to use more than one scrambling systems. The service is scrambled, the encryption system encrypt the key. Finally, different ECMs are transmitted, one for every scrambling system.