By cable box, I am referring to my set-top box with a smart card.
I have a satellite TV connection where I can subscribe to channels, and they update every month.
If I haven’t paid for a specific month, my connection stops working, and I am told to pay on my TV screen.
From what I have been told, the connection is mainly one way – the satellite is broadcasting information to my receiver, and it broadcasts the same signal to everyone, and my set top box can decode it using its smart card, which contains some sort of encryption key.
If the same signal is sent to everyone, is the encryption key the same? And how does my set-top box know which channels I have subscribed to? Is every user’s list of channels broadcasted to everyone?
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The video signals are encrypted using one content key, or maybe one content key per channel.
The content keys are never directly broadcasted, instead the key package is encrypted with different user keys and also included in the broadcast signals, or it can be sent via a bidirectional side channel when the cable box requests it.
In this way the key package is different for every user, but the extracted content keys are the same so the video content doesn’t have to be different for every user, saving a great deal of bandwidth.
Your key package is only being included in the signals if you pay for it. The smart card is your user identity and contains the unique keys to decrypt your key package and extract the final content key. In this way you cannot decrypt other people’s key packages to mooch cable easily.
All the keys are rotated periodically so you have to keep paying to see the content.
I’m not sure if that is the exact implementation, but that’s how I would have designed it.
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