A method for conducting an electronic transaction that includes initiating an electronic transaction between a mobile device and a POS reader device and applying at least a user identifier that is associated with a user of the mobile device and shared secret data as inputs to a cryptographic function in the mobile device. The method further includes inserting the output of the cryptographic function into legacy transaction formatted data, wirelessly providing the legacy transaction formatted data to the POS reader device via the electronic transaction, forwarding the legacy transaction formatted data to the issuer server, and processing, at the issuer server, the legacy transaction formatted data to verify the user identifier and request authorization for the electronic transaction. The method also includes sending a notification message to the POS reader device that indicates that the electronic transaction is authorized and conducting the electronic transaction upon receipt of the notification message.