"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. $