When it comes to charging for content and protecting the associated revenues, the basic requirement was (and still is) access control, which was achieved by employing a technology called conditional access system. But what is conditional access you ask? Well, conditional access is a system that content providers use to protect and monetize their content by controlling who has access to it. It is commonly used on satellite or cable pay-TV services to regulate the contract between provider and user.
One of the main attributes of traditional (legacy) broadcasting is that it is a one-way distribution system. Under this system, the satellite or cable feed is distributed to everyone, and the transmitting organization does not receive any feedback regarding who is picking up their feed or what they are doing with it.
The internal structure of ECMs is proprietary per CAS vendor, but the mechanism of inserting ECMs into the MPEG Transport Stream is standardized through DVB Simulcrypt. DVB Simulcrypt is an interface specification followed by all CAS companies and is integral to how conditional access systems work in general.