Conditional Access
Conditional Access
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It has been suggested that Conditional access system be merged into this article or section. Discuss
Conditional Access abbreviated CA is the protection of content by requiring certain criteria to be met before granting access to this content. The term is commonly used in relation to digital television systems most notably satellite television.
Conditional access in DVB
Under the DVB conditional access system standards are defined in the specification documents for DVBCA Conditional Access DVBCSA the Common Scrambling Algorithm and DVBCI the Common Interface. These standards define a method by which a digital television stream can be obfuscated with access provided only to those with valid decryption smart cards. The DVB specifications for Conditional Access are available from the standards page on the DVB website.
This is achieved by a combination of scrambling and encryption. The data stream is scrambled with an 8byte secret key called the control word. Knowing the value of the control word at a given moment is of relatively little value because it changes very frequently about every 10 seconds. The control word is generated automatically and randomly in such a way that successive values are not predictable; the DVB specification recommends using a physical process for that.
In order for the receiver to unscramble the data stream it must be permanently informed about the current value of the control word. In practise 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 perhaps at monthly intervals. This being apparently not sufficient to prevent unauthorized viewing TPS has lowered this interval down to about 12 minutes.
The contents of ECMs and EMMs are not standardized and as such they depend on the conditional access system being used.
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. 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.
The decryption cards are read and sometimes updated with specific access rights either through a Conditional Access Module CAM a PC cardformat card reader meeting DVBCI standards or through a builtin ISO 7816 card reader such as that in the Sky Digibox.
Several companies provide competing CA systems; VideoGuard Irdeto Access Nagravision Conax Viaccess Latens Verimatrix and Mediaguard a.k.a. SECA are among the most commonly used CA systems.
Due to the common usage of CA in DVB systems many tools to aid in or even directly circumvent encryption exist. CAM emulators and multipleformat CAMs exist which can either read several card formats or even directly decrypt a compromised encryption scheme. Most multiple format CAMs and all CAMs that directly decrypt a signal are based on reverse engineering of the CA systems. A large proportion of the systems currently in use for DVB encryption have been opened to full decryption at some point including Nagravision Conax Viaccess and Mediaguard v1.
Conditional access in the USA
In the United States cable systems the standard for conditional access is provided with CableCARDs whose specification was developed by the cable company consortium CableLabs. Cable companies in the US are required by the Federal Communications Commission to support CableCARDs but standards exist only for one way communication and do not apply to satellite television. Next generation approaches in the United States eschew such physical cards and employ schemes using downloadable software for conditional access such as DCAS. The main appeal of such approaches is that the access control may be upgraded dynamically in response to security breaches without requiring expensive exchanges of physical conditional access modules. Another appeal is that it may be inexpensively incorporated into nontraditional media display devices such as Portable media players.
See also
Access control the same principle applied outside of television.
CableCARD
Card sharing
Compression Networks
Conax
Digicipher 2
Digital rights management
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