How does my cable box know when I have paid?


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?

In: 2

As far as I understand it, they send multiple signals meant for one card at a time. Each card (which has a microchip that can perform computing and store data) listens until it gets a signal it can decrypt, then it follows the orders in the signal. The orders include new decryption keys to use for the channels you subscribe to. Everything is encrypted and how it works is largely kept secret to make hacking the systems (of which there are many) more difficult.

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.

It depends on the implementation for your specific company, with some of them they rely on a literal phone call data transmission to renew your encryption key, others are bouncing the data directly off the satelite to update your box. Satelite TV in itself is a similar principal to cable, where the transmission to everyone is identical, and the box itself is what’s needed for viewing the signal, and in your case, identifying that you’ve subscribed to the specific channel you’re trying to view at that time.

This is being replaced more and more, with the “video on demand” style of box, which is using a system more similar to the internet, to request for download, what ever your watching. This is easier to secure, as your box itself is specifically having to ask for what it wants, and the signal is then sent to it. Some companies use a hybrid of these systems for live/VoD channels and transmissions.