[linux-dvb] Re: TT-budget+ Smart card reader + PIII 700?

[Ralph Metzler <rjkm@convergence.de>]

Hi,

Giuseppe La Rosa writes:
 > Preamble: I don't have a DVB-s card, i follow this list by many months
 > 
 > I don't know how exactly is the TT-budget but i think that it has at least
 > the VES1893 chip, i guess.
 > I'have found here
 > http://www.semiconductors.philips.com/acrobat/other/cms/ves1893.pdf
 > that in this chip there is a DVB descrambler.
 > If this is right, and it really implements the DVB Common scrambling
 > algorithm, it could be used to implement simple, public available, CA system
 > such as the 'Remotely Keyed Encryption Schemes' of cryptolabs. What is
 > missing is to manage the ECM and EMM of this CA system, but i this that this
 > could be done in software.


Yes, it has a DVB descrambler built-in.
No, it does not have a DVB Common Scrambling Algorithm built-in.

The "DVB descrambler" built into the VES1893 (and all other DVB
decoders) refers to the energy dispersal process which is part of the
DVB standard. 
To avoid long series of 0s and 1s in the signal, which could lead to
unwanted interference effects, the DVB stream is pseudo-randomized for
broadcast. This could be described as scrambling with a publically 
known key.

This energy dispersal scrambling/descrambling has NOTHING to do with
the Common Scrambling Algorithm.
Please look it up in ETS 300 421, section 4.4.1 
"Transport multiplex adaptation and randomization for energy dispersal"
where you will find a diagram of the scrambling algorithm.

The VES1893 basically implements all of ETS 300 421 after the
actual reception of the signal (which is done by the rest of the
tuner), nothing more, nothing less.


The Common Scrambling Algorithm (CSA) is usually built into the
demultiplexer, i.e. in parts of the AV7110 in case of the non-budget
card, or into CI-CAMs.
Since the budget card does not have a hardware demultiplexer this
would have to be done in software. They only give the CSA specs to
hardware companies though. Looks like they are not as stupid as the DVD
people ...


Alternatively, one could connect a PCMCIA interface to the PC, plug 
in a CI-CAM (+ official smartcard and payed subscription course) 
and somewhow pass the transport stream through it.
If you look at the PIN descriptions of the CI-CAMS you will notice
that you will have to clock the bytes through the CI-CAM "by hand",
i.e. you will have to set and poll the data valid and other status
pins in software.
Have fun!


Ralph





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