Cable TV Systems Illustrated - pág.35

secret codes.

Content owners (such as movie studios and television networks) commonly
require broadcasters to use CAS and guarantee that content distributed
through their systems will only be delivered to authorized users and cannot
be viewed or copied by unauthorized people.

To control access to content, the media that is broadcasted or streamed is
modified (scrambled or encrypted). For recipients to be able to convert the
media into its original form, the receiver needs to have keys and other information to decode the media. The encryption codes change over time (typically every few seconds). To receive the necessary decoding information, an
additional stream (channel) of decoding information (entitlement messages)
is transmitted along with the broadcasted media.

Content protection systems may also define the content access rules such as
if the received media is authorized to be copied or what types of devices can
view the media (such as standard or high definition televisions). Conditional
access systems may be combined with additional content protection systems
(such as home media distribution).

A downloadable conditional access system (DCAS) is a security process that
is used in a communication system (such as a broadcast television system)
to limit the access of media to authorized users that can be modified or
updated.

Figure 1.20 shows that content protection in the home is an extension of content protection from other sources, including broadcast and stored media
systems. The content from TV broadcasters is protected by conditional
access (CA) systems through a set top box (STB). Content from stored media
(DVDs, Blu-Ray) is protected by a content scrambling system. Content that
is distributed through the home is protected by digital transmission content
protection (DTCP) and high definition content protection (HDCP).