This specification describes a multi-terminal data processing system having means and process for verifying the identity of subscribers to the system. Validity of a terminal request for communication with the data processing system are determined on the basis of a centralized verification system. Each subscriber to the system is identified by a unique key binary symbol pattern. The central data processing unit contains a listing of all valid keys for subscribers to the system.Two embodiments of the centralized verification system are presented, a password system and a handshaking system. In the password system, all data or information originating at the terminal under use of the subscriber is enciphered in combination with the unique subscriber key. Upon proper deciphering of the key or password at the central processing unit and arriving at a match with one of the keys in the processor's listing, the subscriber may communicate with the processing system. In the handshaking system embodiment, the user and the central processor exchange a plurality of messages each formed by a combination of new and prior received data. Received data messages are also maintained within the registers at both the terminal and the central processor for further verification upon the return of the portion of the message that was previously transmitted.