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.

Users

Broadcaster Market Medium
British Sky Broadcasting United Kingdom, Ireland SES' Astra satellites
The Adult Channel Europe SES' Astra satellite
JSTV Europe SES' Astra satellite
SKY Network Television New Zealand terrestrial UHF (Ended 10 Mar 2010)
Sky Fiji Fiji terrestrial VHF
MTV Europe Europe SES' Astra satellite
Foxtel (until 2005) Australia Telstra terrestrial HFC

Versions

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.

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 (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.

The VideoCrypt-S variant, used by the BBC Select service, was based on line shuffle scrambling. This form of video scrambling changes the order in which lines are transmitted thus line 20 may be transmitted as line 32. The VideoCrypt-S variant used six blocks of forty seven lines per field. It had three scrambling formats: full shuffle in which 282 lines were affected; half shuffle, in which every alternate field was scrambled; and a line delay scramble in which the start position of the video in each line was pseudo-randomly delayed.

Attacks

The VideoCrypt system was far from secure and a number of hacks were employed.

Although, the analog UHF option is still available, the introduction of Sky Digital has ameliorated these issues significantly, as the VideoGuard system employed by SkyDigital has not been widely defeated, as of 2009.

Card attacks

Datastream attacks

Decoder card datastream attacks

Brute force

References

This article is issued from Wikipedia - version of the 3/18/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.