The present invention relates to a method and system for securing a validation process of an online payment transaction. During an online transaction on a mobile device, it is requested a contactless tap from a contactless display card (thanks to NFC reader mode in the mobile device) to establish a contactless exchange session. During this contactless exchange session, the display card receives the amount requested for the transaction and the identification of merchant. The user is invited to check amount and merchant identification on the screen of the display card. When the user validates the transaction by for example pressing on a corresponding button then the display card creates an authorization message comprising enciphered financial user information and data on the transaction. With a second tap from the display card, the mobile device receives the authorization message which is transmitted to the display card issuer through the merchant for validation.