Abstract: Methods and systems for secure transaction authentication. The card is read in a POS reader, data is sent to a remote server after being encrypted, a code is then sent from the remote server to a unique identifier provided as a proof of authentication, thereafter the code is provided to authenticate the transaction. The methods and systems may include obtaining a unique identifier from the user, such as a mobile telephone number. In some configurations, the data of the mobile telephone number can be compared to patterns of activity related to usage of the mobile telephone number. The methods and systems may include sending a code that is provided by during the transaction and verifying that the code provided matches with the code sent by the remote server.