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. Basic operation The security of conditional access smart card technology is limited by the security of the DVB standard in which it operates. The standard practice of a legitimate smart card is to decrypt an ECM (Entitlement Control Message), which then provides the control word, which allows the viewing of scrambled material. With card sharing, however, the smart card and its security features are bypassed; software intercepts the decrypted control word and allows the user to share it across a computer network. Pirate decryption Card sharing has established itself as popular method of pirate decryption. 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. Because the length of the complete control word is so small (64 bits), delivery of the control words to many different clients is easily possible on a home internet connection. This has sparked the creation of sharing network groups, in which users can access the group by sharing their subscription cards with the group, and in turn, being capable of receiving the channels which all users' cards can decrypt, as though the user owned every single subscription card connected to the network. Other networks have also been created, whereby one server has multiple legitimate subscription cards connected to