Conditional access system is a technology of used to control access to advanced TV (DTV) services to approved users by scrambling all of the transmitted programming. CAS has been used for years for pay-TV services.
This is achieved by a combination of scrambling and encryption. The information stream is mixed with a 48-bit mystery 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 produced consequently such that progressive qualities are not typically unsurprising; the DVB determination prescribes utilizing a physical procedure for that.
All orders for the collector to unscramble the information stream, it must be for all time educated about the present estimation of the control word. In practice, it must be educated marginally ahead of time, with the goal that no survey interruption occurs. Encryption is utilized to secure the control word amid transmission to the beneficiary: the control word is scrambled as a privilege control message (ECM). The CA subsystem in the recipient will unscramble the control word just when approved to do as such; that expert is sent to the form as an of an entitlement management message (EMM).