1. A method for updating air interface keys, comprising: initiating, by a Mobility Management Entity (MME), an Authentication and Key Agreement (AKA) procedure after a user equipment has performed an inter-system handover; transmitting, by the MME to an evolved NodeB (eNB) through a context modification message, a new key K eNB derived at least in part by a key which is generated during the AKA procedure; and updating, by the eNB, air-interface keys based on the new key K eNB .