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!