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.
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.
Much of the development of card sharing hardware and software has taken place in Europe, where national boundaries mean that home users are able to receive satellite television signals from many countries but are unable to legally subscribe to them due to licensing restrictions on broadcasters.
However, in some cases the contract between the subscriber and the content provider implicitly or explicitly prohibits this kind of card sharing.
Card sharing is a particular concern to conditional access providers, and their respective pay-TV companies, as well as the DVB consortium.
Rather than sending a plain text control word from the smart card to the receiver's microprocessor, which can be intercepted, the decrypted ECM will in fact be an encrypted control word, which can only be decrypted by a legitimate, non card sharing capable, receiver.
[5][6] In August 2009, a Liverpool man was prosecuted for re-selling the services of a card sharing network to paying customers.