Method and apparatus for associating SIM card with a group of mobile communications devices

A mobile communication device comprises a trusted electronic processor coupled to the application electronic processor. The trusted processor receives from a subscriber identity module (SIM) card a hashed value generated based on a random value and a first key. It then generates and sends a response to the SIM card by decrypting the hashed value using a second key, and the SIM card responds with a positive acknowledgment if this is as expected. On receiving this acknowledgment, the mobile device may connect or register to the communications network associated with the SIM card. The system may be used to associate a SIM card with a group of mobile devices, hence allowing the card to be used with any device within the group whilst preventing use on unauthorised devices. A facility to disable this security feature may also be provided, in which a signal sent by a network operator prompts the SIM card to give zero as the hashed value and always respond with a positive acknowledgement.