Abstract: A time-shared central computer can be protected from access from unauthorized users by utilizing automatic tracing equipment included within telephone switching systems, in combination with an independently organized offsite security computer system. The user, seeking to access the central computer through a telephone system, automatically causes his telephone number to be generated within the telephone system and sent to the offsite security computer. Meanwhile, the user provides the time-shared, central computer with a password which is used to address a memory which includes file information on all authorized users. The file information which includes the telephone number of all authorized users, is made available in the offsite security computer for comparison with the telephone number corresponding to the calling user as provided by the telephone switching system. A match indicates that a user, in possession of preauthorized password, is calling from appropriate telephone station.