VideoCrypt

VideoCrypt is a cryptographic, smartcard-based conditional access television encryption system that scrambles analogue pay-TV signals. It was introduced in 1989 by News Datacom and was used initially by Sky TV and subsequently by several other broadcasters on SES' Astra satellites at 19.2° east.

Three variants of the VideoCrypt system were deployed in Europe: VideoCrypt I for the UK and Irish market and VideoCrypt II for continental Europe. The third variant, VideoCrypt-S was used on a short-lived BBC Select service. The VideoCrypt-S system differed from the typical VideoCrypt implementation as it used line shuffle scrambling.

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 (PRNG) 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 card's PRNG with a seed transmitted with the video signal to generate the correct sequence of cut points.

The system also included a cryptographic element called the Fiat Shamir Zero Knowledge Test. This element was a routine in the smartcard that would prove to the decoder that the card was indeed a genuine card. The basic model was that the decoder would present the card with a packet of data (the question or challenge) which the card would process and effectively return the result (the answer) to the decoder proving that it was a genuine card without disclosing any critical information. If the decoder received the wrong result from the card, it was supposed to stop decoding the video. However a technologically insecure implementation of this otherwise strong cryptographic element made it redundant.

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