[NOISE] Cable-TV-Piracy-Punks

[mpd@netcom.com (Mike Duvos)]

"David K. Merriman" <merriman@arn.net> writes:

 > At 01:34 AM 03/28/96 +0000, you wrote:

 >> I've been looking for a file on how to make PPV
 >> descramblers and havn't found any. Commercial descramblers
 >> cost around $200 base price. If anyone has a file on how to
 >> make them please e-mail me one.  Thanks.

 > This is cypherpunks. Not Cable-TV-Piracy-Punks.

ObCrypto: Scrambling TV signals sometimes makes use of
encryption, so perhaps a brief discussion of how this is done
could be tolerated.

If you are talking about recovering signals from completely
encrypted digital MPEG-2 streams, such as those used by the DBS
folks, you are probably out of luck.  The relevant processing in
the decoder exists on a small card which has so far resisted
attempts at reverse engineering.

As far as analog signals are concerned, the "scrambling" of the
video only involves the clipping of the horizontal sync pulses.
This causes the picture to tear and the color burst to be missed.
So you get a funny torn picture with odd colors in place of the
original.  Sometimes, the set will momentarily lock on to
something in the picture in place of the missing sync and you
will get a reasonable picture for a few moments.  Kids often
watch porn channels for hours waiting for such an effect to
occur.

The usual way of transmitting the missing sync information is to
place an appropriately tuned 15,750 hz subcarrier on the sound
channel.  If you pick this up, and use it to add pulses back onto
the video, you will again get a signal your set will correctly
process.

There are a variety of techniques for scrambling audio.  The most
expensive is to DES encrypt the sound and place it in the
horizontal blanking interval.  The regular sound channel can then
be used for advertising.  This requires a bit of processing at
both ends, and is generally used for satellite to ground
transmission of cable signals.  The other common method is to
modulate the sound on a subcarrier, usually the one transmitted
in phase with the missing sync.

In most cable systems using addressable decoders, nothing is done
to the sound at all, and the box simply mutes the set if its
address is not in the list of authorized users for that channel.
Persons viewing a PPV without a cable box will get a scrambled
picture, but perfectly normal sound.  This is commonly referred
to by people who listen to PPV events without paying as watching
in "scramblevision."

A hostile attack on such a system can be mounted in a number of
ways.  One can simply mung the set top box to restore sync on all
channels unconditionally, either by replacing a single chip with
a black market substitute, or by doing some surgery on the
electronics.  Once can also construct a number of simple circuits
which will yank the subcarrier off the sound channel, and use it
to trigger a pulse generator which gets added to the video.  You
can even stick one of these in your TV set to render it truly
"cable ready."

Of course, once television transmission goes completely digital,
and strong encryption is used on both audio and video, the
opportunity for such simple attacks will vanish.

--
     Mike Duvos         $    PGP 2.6 Public Key available     $
     mpd@netcom.com     $    via Finger.                      $