A method for deriving keys is disclosed. When Handover or Routing Area Update of User Equipment (UE) from an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) to a Universal Terrestrial Radio Access Network (UTRAN) or Global System for Mobile Communication/Enhanced Data Rate for GSM Evolution Radio Access Network (GERAN) occurs, the keys for the UTRAN or the GERAN are derived by a Mobility Management Entity (MME) 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 (NAS) Count. A keys derivation system for deriving the keys is also disclosed.