The system comprises a terminal (10, 12), an independent portable device (20) including a data processing means, and a wireless coupling means (RF COMMUNICATION) for exchanging individual-identification data between said terminal and said portable device. A body-medium communication means (OSC COMMUNICATION) including a transmitter in the terminal and a receiver in the portable device is provided to transmit from the terminal to the portable device a connection code (CONNECTION CODE) at the onset of a transaction upon physical contact established by the individual between the terminal and the portable device. A control means in the portable device checks said connection code received and conditionally issues to the terminal through said wireless coupling means (RF COMMUNICATION) a signal for enabling further execution of said transaction in response to said connection code complying with predetermined criteria.