Conditional Access

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What Does Conditional Access Mean?

Conditional access (CA) is an access control method used in digital television transmissions that restricts what viewers can watch. Generally, it refers to the type of access a service provider allows for its customers, which is often limited only to the services that the latter have subscribed to. CA is implemented with the help of scrambling and encryption algorithms. It essentially prevents unauthorized usage of services by a consumer.


Techopedia Explains Conditional Access

Conditional access is a technology used in digital transmissions that limits the content that can be viewed by a customer. It is thus useful in subscription services like digital video broadcasts and television broadcasts where customers are allowed to view only the channels they have paid for. CA is used by broadcasting companies to ensure the authorized usage of their services.

Some of the basic components of CA are as follows:

  • Subscriber management system
  • Subscriber authorization system
  • Security module
  • Set-top box

When a broadcast equipment transmits encrypted data to the user, the set-top box filters these data signals and passes them to the security module. The security module checks the authorization status of the data received and, if authorized, decrypts the data and allows display to the user.

CA systems use the Digital Video Broadcasting protocols such as SimulCrypt and MultiCrypt.

SimulCrypt uses more than one set-top box, whereas MultiCrypt allows multiple CA systems to be used with a single set-top box containing a smart card for each CA system used.

CA is implemented using a combination of scrambling and encryption techniques. A 48-bit secret key known as the control word is used to scramble the broadcast data, and this control word is changed frequently to avoid hacking. The control word is protected using encryption during transmission to the receiver as an entitlement control message. The CA subsystem present in the receiver’s set-top box can only decrypt the control word if it has been authorized to do so via an entitlement management message (EMM). An EMM is unique for each user and is identified by the user’s smart card.


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Margaret Rouse
Technology Expert
Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.