Abstract: A security system is disclosed for use in an electronic funds transfer environment. The system includes a central processor and a portable transaction device. During initiation, the central processor generates a sequence of transaction numbers and associated random numbers. This data is supplied to and stored in the portable transaction device. In operation, the next available random number is determined and combined with a transaction parameter to generate a security code. The security code may be verified immediately or during the check clearing cycle at the central processor. In the preferred embodiment, in the on-line operation, the selection of the ramdom number is altered by a random offset number further enhancing security.