a method for deriving keys is disclosed . when handover or routing area update of user equipment from an evolved universal terrestrial radio access network to a universal terrestrial radio access network or global system for mobile communication / enhanced data rate for gsm evolution radio access network occurs , the keys for the utran or the geran are derived by a mobility management entity and / or the ue by using predefined parameters . the predefined parameters include a root key of the e - utean and a value of a non access stratum count . a keys derivation system for deriving the keys is also disclosed .