A system for the authentication by a card-issuing financial institution of identifying information of a card-holding user of a public data network. The system includes a secure data entry device connected to the public data network and a gateway device connected to the public data network and to a private data network used for transmitting messages between financial institutions. The secure data entry device enables the user to enter identifying information of a card issued by the card-issuing financial institution, and transmits the identifying information in a secure manner over the public data network to the gateway device. The gateway device transmits the identifying information to the card-issuing financial institution and receives an approval response from the card-issuing financial institution over the private data network. The approval response provides authentication of the identifying information by the card-issuing financial institution.