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When setting up conditional access, access can be limited to or prevented based on the policy defined by the [[system administrator]]. For example, a policy might require access is available from certain networks, or access is blocked when a specific [[web browser]] is requesting the access.
==In digital television==
Under the [[Digital Video Broadcasting]] (DVB) standard, conditional access system (CAS) standards are defined in the specification documents for DVB-CA (conditional access), [[Common Scrambling Algorithm|DVB-CSA]] (the common [[Scrambler|scrambling]] algorithm) and [[DVB-CI]] (the [[Common Interface]]).<ref>{{Cite web |title=Security |url=https://dvb.org/solutions/security/ |access-date=2022-12-05 |website=DVB |language=en-US}}</ref> These standards define a method by which one can obfuscate a digital-television stream, with access provided only to those with valid decryption [[smart cards|smart-cards]]. The DVB specifications for conditional access are available from the [https://web.archive.org/web/20130116162443/http://www.dvb.org/technology/standards/index.xml#conditional standards page on the DVB website].
This is achieved by a combination of [[scrambler|scrambling]] and [[encryption]]. The data stream is scrambled with a 48-bit secret key, called the ''control word''. Knowing the value of the control word at a given moment is of relatively little value, as under normal conditions, content providers will change the control word several times per minute. The control word is generated automatically in such a way that successive values are not usually predictable; the DVB specification recommends using a physical process for that.
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In order for the receiver to unscramble the data stream, it must be permanently informed about the current value of the control word. In practice, it must be informed slightly in advance, so that no viewing interruption occurs. [[Encryption]] is used to protect the control word during transmission to the receiver: the control word is encrypted as an ''entitlement control message'' (ECM). The CA subsystem in the receiver will decrypt the control word only when authorised to do so; that authority is sent to the receiver in the form of an ''entitlement management message'' (EMM). The EMMs are specific to each [[subscriber]], as identified by the smart card in his receiver, or to groups of subscribers, and are issued much less frequently than ECMs, usually at monthly intervals. This being apparently not sufficient to prevent unauthorized viewing, [[Télévision Par Satellite|TPS]] has lowered this interval down to about 12 minutes. This can be different for every provider, [[British Sky Broadcasting|BSkyB]] uses a term of 6 weeks. When [[Nagravision|Nagravision 2]] was hacked, [[Digital+]] started sending a new EMM every three days to make unauthorized viewing more cumbersome.
The contents of ECMs and EMMs are not standardized and as such they depend on the conditional access system being used.<ref>https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.1852-1-201701-I!!PDF-E.pdf</ref>
The control word can be transmitted through different ECMs at once. This allows the use of several conditional access systems at the same time, a DVB feature called ''simulcrypt'', which saves bandwidth and encourages multiplex operators to cooperate. [https://web.archive.org/web/20130116162443/http://www.dvb.org/technology/standards/index.xml#conditional DVB Simulcrypt] is widespread in Europe; some channels, like the [[CNN International]] Europe from the [[Hot Bird]] satellites, can use 7 different CA systems in parallel.
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