Conditional Access TV
A Conditional Access System (CAS) is one which provides users the option of subscribing to particular services of their choice from the gamut of packages offered by the provider and pay only for those services they have opted for. This term is prevalent in television broadcasting, where, formerly, users could only receive the complete array of television channels supported by the provider and could not customize their packages.
To provide this 'conditional access' to services to the subscriber base, cryptography is employed. During transmission, a control key is used to scramble the content to be broadcast. This scrambled content is transmitted over the transmission medium and the receiver, usually called a set-top box, then has to unscramble the data to get the original content intended to be broadcast. The service provider distributes smart card modules that can be plugged in to the subscribers' set-top boxes. These smart cards are customized to allow the descrambling of the channels in the package the subscriber has opted for and prevent access to other channels.
The descrambling process requires the same control key that was used to scramble the original data at the transmitter side. Hence, this control key has to be transported to the receiver. Sending the control key as is would defeat its purpose since it could then be tapped and used to recover the signal. So, the control key is encrypted with a cryptographic encryption algorithm and transmitted alongside the scrambled content as an “Entitlement Control Message” (ECM). The receiver decrypts this to obtain the control key and then uses it to get back the original signal. The receiver also requires an “Entitlement Management Message” (EMM), which authenticates it to perform the decryption. To increase the level of security, the control key is changed continually – every 10 seconds, for example.
Unsurprisingly, the Internet has not 'spared' Conditional Access System! The new Downloadable Conditional Access System provides for the software on the receiver to be downloaded dynamically via the Internet. Though this system provides a more flexible and easily updatable solution, a train of other security issues entail its implementation.
http://www.freshpatents.com/Conditional-access-system-dt20071108ptan20070261076.php
http://blogs.cisco.com/sp/2007/09/dmystifying_dcas_downloadable.html
Reference
- Conditional Access (CA)
- http://www.dtg.org.uk/reference/tutorial_ca.html
- Fresh Patents - Conditional access system
- http://www.freshpatents.com/Conditional-access-system-dt20071108ptan20070261076.php
- D-mystifying 'DCAS'
- http://blogs.cisco.com/sp/2007/09/dmystifying_dcas_downloadable.html
- Image: Copyright 1997-2008 The Digital TV Group Limited, Used with permission
- http://www.dtg.org.uk/reference/images/ca_block.gif
