Abstract: Security of communications in a packet-switched data communications system is enhanced by introducing terminal and host security devices into the system in communicative relationship with a terminal and a host processor, respectively. In response to a user-initiated data entry at the terminal, the terminal security device generates an initial data packet indicative of user authorization or not, but which is unsuited for processing by the addressed processor, ahead of additional data packets containing user-entered message data to be processed by the addressed processor. The host security device intercepts and processes the initial data packet and, if user authorization is indicated therein, replaces it with an artificial data packet solely to render the additional packets amenable to processing by the addressed processor and thereby to establish a communications session between user terminal and processor-associated database to which access was requested.