The disclosure relates to a security method in a radio access network system. A shared secret key is stored in both a user device and a core network system. A further secret key is received from the core network system, wherein the further secret key has been derived using the shared secret key stored in the core network system. One or more values are provided over the radio interface to the user device to derive the further secret key in the user device from at least the shared secret key stored in the user device and one or more of the one or more values provided over the radio interface. An authentication procedure and/or a key agreement procedure is performed for the user device over the wireless radio interface using the received further secret key in the radio access network system and the derived further secret key in the user device.