Systems, apparatuses, and methods for increasing the security of electronic payment transactions, such as eCommerce transactions conducted over the Internet. A transaction approval or authorization mechanism uses an out of band process to provide authentication or identification data that has previously been registered by a user and associated with the user's payment device or account. The out of band authentication data may be provided in response to a message sent to a user's mobile phone, where the message is generated in response to entering the user's phone number into a form that is provided when the user engages in an electronic payment transaction using a desktop computer. The data may include a digital signature and associated digital certificate that is used to authenticate the user.