Information AboutVideocrypt |
| CATEGORIES ABOUT VIDEOCRYPT | |
| satellite television | |
| digital rights management | |
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Two variants of the VideoCrypt system were deployed in Europe: VideoCrypt I for the UK and Irish market and VideoCrypt II for continental Europe. OPERATING PRINCIPLE The system scrambles the picture using a technique known as Line Cut-and-Rotate. Each line that made up each picture (video frame) is cut at one of 256 possible "cut points", and the two halves of each line are swapped around for transmission. The series of cutpoints is determined by a pseudo-random sequence. Channels were decoded using a Pseudorandom Number Generator sequence stored on a Smart Card (aka Viewing Card). To decode a channel the decoder would read the smart card to check if the card is authorised for the specific channel, if not a message would appear on screen, otherwise the decoder seeds the cards PRNG with a seed transmitted with the video signal to generate the correct sequence of cut points. ATTACKS The VideoCrypt system was far from secure and a number of hacks were employed. Hackers discovered methods of preventing Sky from killing their cards, these worked until Sky replaced the cards. Hackers also discovered ways of switching on "dead" cards using a computer. Other successful hacks involved sampling the datastream between the card and the decoder, for example you could record a movie and store the decoder information so that people could then use it to decode the same movie that they recorded earlier with a decoder and "dummy" card. This worked because the decryption seeds were recordable on normal VCR's. The most successful hack on the VideoCrypt system is the "McCormac" hack devised by John McCormac, it involved broadcasting the decoder-card data live and other decoders could use it to watch the encrypted channels effectivly sharing a card with several decoders. Furthermore, as desktop computing power increased, such a simple system was always inherently vulnerable to brute force 'image-processing' attacks; even in the absence of any information about the cutpoint sequence, adjacent lines in a picture can be 'correlated' to find the best match, and the picture reconstructed. This will not work for all pictures, but is an interesting proof-of-concept. Markus Kuhn 's Antisky.c program from 1994 is an early example of such an attack. More recently it has been shown that, using detailed knowledge of the way colour is transmitted in analogue TV systems, 'perfect' reconstruction could be achieved for many scenes. This all changed when Sky discontinued its analogue service in favour of Sky Digital which uses the VideoGuard system which so far has not been beaten. REFERENCES
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