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Concept
Common Interface
Summary
In Digital Video Broadcasting, the Common Interface (also called DVB-CI) is a technology which allows decryption of pay TV channels. Pay TV stations want to choose which encryption method to use. The Common Interface allows TV manufacturers to support many different pay TV stations, by allowing to plug in exchangeable conditional-access modules (CAM) for various encryption schemes.
The Common Interface is the connection between the TV tuner (TV or set-top box) and the module that decrypts the TV signal (CAM). This module, in turn, then accepts the pay-to-view subscriber card, which contains the access keys and permissions.
The host (TV or set-top box) is responsible for tuning to pay TV channels and demodulation of the RF signal, while CAM is responsible for CA descrambling. The Common Interface allows them to communicate with each other. All Common Interface equipment must comply with the EN 50221-1997 standard. This is a defined standard that enables the addition of a CAM in a DTV receiver to adapt it to different kinds of cryptography. The EN 50221 specification allows many types of modules but only the CAM has found popularity because of the pay TV market. Indeed, one of Digital Video Broadcasting's main strengths is the option of implementing the required conditional access capability on the Common Interface.
This allows broadcasters to use modules containing solutions from different suppliers, thus increasing their choice of anti-piracy options.
A DVB receiver may have one or two slots implementing the Common Interface (CI). The CI uses the conditional-access module (PCMCIA) connector and conforms to the Common Scrambling Algorithm (CSA), the normative that specifies that such a receiver must be able to accept DES (Data Encryption Standard) keys in intervals of some milliseconds, and use them to decode private channels according to a specific algorithm.
Those algorithms are proprietary to individual suppliers. Each one uses their own algorithms and there is no defined standard for them.
Official source
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