[vdr] encrypted channels - dumb question

Nicolas Huillard nhuillard at e-dition.fr
Thu Oct 20 23:40:02 CEST 2005


Damien Bally a écrit :
> Hello
> 
> As far as I know there is a plugin named sc (softcam) that allows someone to
> view encrypted channels.
> Can anyone explain the way it works ?
> Don't misunderstand me : I don't want an howto to do something illegal, just
> some theory to satisfy my curiosity.

SC replaces the hardware CAM, when you have a budget card with no CI
(connector for the CAM). It can be a fully software CAM, or use a
smartcard reader to use the subscription card you have.
Another missing part in common budget card is the descrambler chip,
which can be replaced by a software one. The scrambling algo is called
CSA (common scrambling algorithm).
I don't know what can be considered illegal there, except when you do
not use  a valid subscription card. There is less in all this than in
DeCSS, which includes keys (ie. smartcard) + key management (ie. SC) +
algo (ie FFdeCSA).

The theory behind that is roughly :
* the stream contains information about key management algo
* it also contains random bytes (like a public key)
* the CAM must use one of it's internal key management algo, along with
those bytes, to select the appropriate private key inside the smartcard
* it then transmits the key to the descrambling chip, that does the
actual descrambling
* the whole process is repeated at each channel change, and also from
time to time, I think

I hope I didn't upset anyone with that...

-- 
NH



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