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.

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 it. Access to this server is then restricted to those who pay the server’s owner their own subscription fee.

One technical method, implemented by providers such as Irdeto and NDS, is to update the software of digital receivers provided by the subscription television service. This software implements a further decryption layer, held within the receiver. 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. A simpler method, used by several providers, is to simply increase the frequency of control word changes. With changes occurring as frequently as once every few seconds, extra stress is put onto the smart card sharing system, meaning that clients may be frustrated by short, frequent, missed viewing periods.