The invention relates to a method for improving the security of authentication procedures in digital mobile radio telephone systems. In order to make it more difficult if not impossible to work out a secret code KI, several different secret SIM-specific codes KI are contained in the mobile radio telephone network and on a subscriber identity module SIM and a code KI for the implementation of said authentication is selected from the various secret codes thus contained during the authentication process between the subscriber identity module and the mobile radio telephone system pertaining to said SIM.