Abstract: A method for authenticating a transaction between an initiator device and a transactor device over a data network, according to which a transaction request is submitted to the transactor device over the data network and an initiator determined one time parameter (OTP) is generated, based on parameters that are associated with the transaction request and with initiator activity. The initiator determined OTP is compared with a non-initiator determined OTP, both generated by means of an identical OTP engine. The transaction is denied if the initiator determined OTP and the non-initiator determined OTP are found to be different. The initiator activity is interfacing with a puzzle that is randomly selected and displayed on the initiator device, where the OTP engine generates an OTP as a function of parameters of the transaction request and of a puzzle result associated with the puzzle transmitted to the initiator device.