Abstract: A method for efficiently protecting transactions and providing authentication of users and devices in on-line systems that transfer funds electronically, dispense cash, or provide a good or permit a service to be utilized is provided. The transaction may be initiated by a magnetic-striped plastic card at an attended or unattended terminal (10, 11, 12) and requires the entry of a preassigned Personal Identification Number through a keyboard (20). The Personal Identification Number is encrypted (23) more than once at the terminal and other means are used in order to prevent the utilization of certain tapped-line data. The data required to validate and authorize the transaction is transmitted securely to a centralized computer (14) which accesses from its stored data base (15) the data that is required to decrypt and validate the transaction, including the encrypted Personal Identification Number corresponding to the received transaction data.