According to an aspect of the present invention there is provided a method of operating a communication device, the communication device being part of a group comprising two or more communication devices that share a subscription to a communication network. The method comprises receiving a group authentication challenge from the network, at least part of the group authentication challenge having been generated using group authentication information that is associated with the shared subscription. The device then generates a device specific response to the group authentication challenge using the group authentication information and device specific authentication information and sends the device specific response to the network. The device is for example a member of a machine-type communication device group.