Card sharing, also known as control word sharing, is a method of allowing multiple clients or digital television receivers to access a subscription television network with only one valid subscription card. This is achieved by electronically sharing a part of the legitimate conditional access smart card's output data, enabling all recipients to gain simultaneous access to scrambled DVB streams, held on the encrypted television network.
Typically, a legitimate smart card is attached to a host digital television receiver, which is equipped with software to share the decrypted 64-bit "control word" key over a computer network, such as the Internet. Once a client receives this key, they can decrypt the encrypted content as though they were using their own subscription card.
The theory of card sharing as a potential attack vector on pay TV encryption was provided in the book European Scrambling Systems by John McCormac in 1996; leading to the term "McCormac Hack" to be used to describe early card-sharing systems.
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