A method for deriving keys comprising: deriving keys for a Universal Terrestrial Radio Access Network (UTRAN) or a Global System for Mobile Communication/Enhanced Data Rate for GSM Evolution Radio Access Network (GERAN) by a Mobility Management Entity (MME) and/or User Equipment (UE) by using predefined parameters when Handover or Routing Area Update of the UE from an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) to the UTRAN or the GERAN occurs, wherein the predefined parameters include a root key of the E-UTEAN and a value of a Non Access Stratum (NAS) Count.
