die@frog.UUCP (Dave Emery) (05/03/88)
The standard scrambling techniques: 1. Sync suppression. There are several versions of this very popular scheme including sine wave, pulse, and tri-level systems. They all work by reducing the amplitude of the synchronizing pulses used to keep the horizontal sweep of the set synchronized with the timebase of the video source so that the sync separator circuits in the TV cannot reliably distinguish picture from sync and the picture tears. Usually a reference signal that can be used to reconstruct the correct sync is transmitted by AM modulating the normally FM sound carrier or as a subcarrier (in over-the-air systems) on the sound carrier. 2. Jamming tones - a strong AM modulated carrier in the lower sideband of the TV signal or near the sound carrier destroys intelligability of picture and sometimes also sound unless a sharp, deep notch filter between the set and cable is used to remove it. 3. Video inversion. This method is rarely used alone, most often is combined with sync suppression. Several of the most popular modern cable and on-the-air scrambling schemes invert the polarity of the video (turning black to white and visa versa) randomly under command of a control bit transmitted in either the vertical blanking interval or as AM on the sound carrier. 4. Digital sound transmission. This scheme combined with sync stripping is reputed to be the basis of the Oak Orion scrambling used to encrypt Canadian satellite TV and various private satellite feeds. I have seen it stated that no encryption of the digital sound information is used (or only a very simple one) , I have not verified this. 5. DES encryption of the sound. This is the method used by the videocipher II system that encodes most cable satellite feeds. The sound is converted into a digital bit stream and encrypted with DES. The picture is inverted, and all sync is stripped (the sync time and vertical blanking interval is used to transmit the digital sound and authorization messages), and the color burst is shifted in level. 6. Time serial transmission of the picture as separate luminence and chrominence components multiplexed in time. The picture is split into 3 separate components which are transmitted one after the other line by line. The starting point of each line is delayed by a psuedo-random amount under control of a DES based key. Audio is sent digitally encoded with DES. This scheme, called B-MAC (Burst-Multiplexed Analog Components) is used to encrypt the Holiday Inn satellite TV feeds and various other satellite services (EG Australian TV). 7. Chopping the video up into little slices and transposing them. under control of a pseudo random key. This scheme (Videocipher I) is used to encrypt certain CBS satellite network feeds. Reconstruction of the transposed video is accomplished by storing the scrambled video into appropriately pseudo-randomly chosen places in a digital frame store. Allegedly the pseudo-random sequence used is based on DES. Sound is DES scrambled. 8. Conversion of the video into a complex digital bit stream using sophisticated compression transformations implemented with high powered specialized signal processors followed by encipherment with DES (or other) encryption. This technique is used by many satellite teleconferencing systems. Sound is also transmitted digitally under DES or via telephone lines. Breaking scrambling Some of these scrambling schemes are very easy to break (and mostly used on older cable systems), others can be broken only via unfortunate weaknesses in the key control and update firmware (such as Videocipher II), and some are presumably nearly unbreakable (depend only on the crypographic strength of DES used in a strong mode). As for the legality of descrambling TV signals, the ECPA of 1986 makes it a felony punishable by 5 years in jail and a $250,000 fine to deliberately intercept a "scrambled or encrypted" signal from an "electronic communications system" if the information on the signal is "not readily accessible to the general public". As of yet no court has interpreted these words, and nobody has been prosecuted for violating this provision of the new law. Section 705 of the Communications Act of 1934 allows reception of unscrambled satellite TV but explicitly makes it illegal to intercept scrambled satellite TV. I am unaware of many actual prosecutions under these provisions either. And most states (including this one - Mass) now have cable theft of services statutes that either explictly or by broad language forbid unauthorized descrambling of cable TV for the purpose of fraudulently obtaining premium services. There have been quite a number of prosecutions around here of dealers who make and sell illegal cable boxes set up to descramble pay services. Users caught with illegal boxes are usually just made to pay for the services they could have received for the entire period they had the illegal box and the box confiscated. ---- David I. Emery Charles River Data Systems 983 Concord St. Framingham, MA 01701 Tel: (617) 626-1102 uucp: ...!decvax!frog!die -- ---- David I. Emery Charles River Data Systems 983 Concord St. Framingham, MA 01701 Tel: (617) 626-1102 uucp: ...!decvax!frog!die