[net.audio] Video descrambling notes

paul@uiucuxc.UUCP (08/11/84)

#N:uiucuxc:18500034:000:2684
uiucuxc!paul    Aug 11 14:36:00 1984

Here are some notes I've collected on de-scramblers:

***** tekcad:net.crypt / eagle!karn /  2:28 am  May  4, 1983
Now that this newsgroup has been created, I'd like to kick off a
discussion on a proposed video scrambling scheme for Home Box Office
satellite transmissions.

The system is being manufactured by M/A-Com Linkabit.  I have sparse
information at present, including their glossy sales brochure.  From
what I can gather, the system appears to have the following characteristics:

1. Video scrambling is done by time-delaying each line according to an
amount determined by a DES sequence.

2. Video sync is removed, making the use of a simple timebase corrector
impossible.

3. Audio is digitized, and then bit-by-bit DES encrypted.  The bits are
placed in the horizontal sync interval (where horizontal sync used to
be.)

A few comments:
1. I already have some ideas for recovering the video without having to
break the DES. Since most video signals have a very high degree of
line-to-line correlation, it may be possible by using line-by-line video
autocorrelation to determine the correct amount of delay for each scan line.

2. I hear that many satellite receiver vendors have vowed to commit whatever it
takes to breaking this system.  Looks like they'll have a real job to
do, and DES may get its most determined test yet.

3. I did not know that high speed DES chips (digitized audio is usually
a megabit/second proposition) were available; perhaps the same chips
could be used in a brute-force key search machine.

Any further details on the VideoCypher would be welcome.

Phil

/**** uiucuxc:net.video / ihu1g!fish /  6:19 pm  May 16, 1984 ****/
Sine wave encoding is used by ON TV and several cable companies.  It
is a very simple encoding scheme which consists of a 15.1 KHz sine
wave algebraically added to the video signal prior to CW modulation.
The scrambled picture usually has an large, unstable wavy line in
the middle of the screen that occasionally moves aside to reveal an
almost-intelligible picture, especially on sets with good AGC sections.
The audio is on a subcarrier, not in the usual audio band.

Pirate decoder kits defeat sine wave encoding by extracting the 15.1 KHz
scrambling signal, inverting it, and feeding it back into the AGC of the
IF section, creating a negative feedback loop which stabilizes the video
signal -- sort of.  The set's or VCR's AGC still plays a role in 
stabilizing the picture.  Picture quality on the pirate decoders that
I've seen ranges from poor to fair, and the sound leaves a lot to
be desired.  
-- 

                               Bob Fishell
                               ihnp4!ihu1g!fish
/* ---------- */

newton2@ucbtopaz.CC.Berkeley.ARPA (08/13/84)

Regarding the problem of real-time highspeed DES encryption of digitized
audio in the MA/com linkabit HBO scrambler: I doubt that the actual audio
bitstream is DES-encrypted in real time; more likely a seed for a family of
scramble tables is transmitted under DES, this key is decrypted at leisure
giving information needed to set up the receiver to correctly transpose the
bits. This just requires slightly faster than realtime cpu instruction
cycles (I mean faster than the audio bit rate); incoming bits are buffered
and read out in the correct sequence  by consulting the table. New tables
can be sent as often as reasonable to spoil the fun for anyone who finally 
cracks a fewe seconds of audio.

I don't know for sure that this is what's being done, but it's what *I* would
do- and have done on projects for clients whose needs are similar to HBO's.

I think the video line transposition works the same way- each frame might
have a different transposition vector, as it were. I agree with you that there'sa lot of redundancy in most images that might be a chink in the armor, but
many systems propose transposing segments of each line as well (or instead).
And realtime autocorrelation among 525 lines of video is a little beyond
the resources of the vendors of pirate TVRO's, methinks.

mikey@trsvax.UUCP (08/13/84)

#R:uiucuxc:18500034:trsvax:55100051:000:535
trsvax!mikey    Aug 13 15:59:00 1984



WARNING!! WARNING!! WARNING!!  DANGER Will Robinson!!!

I thought I'd warn you guys that there are some self proclaimed
guardians of information on the nets!  Be careful!  You are no longer
safe!  They will track you down to the ends of the phone lines and
FLAME at you!!!  

Be prepared for:
	
	Warnings of moral laws broken
	Warnings of man-made laws broken
	Warnings ...  (Insert appropriate topic here)
	*ssholes

Beleive me, I know.  


mikey  (They already know where I am, and I can't run)

Trust me, these views are my own!