The present invention generally relates to systems and methods for authenticating transactions through a simplex communication. To allow trusting the transaction with a payment by an image code such QR code, the invention proposes means to authenticate said transaction. For that, when the mobile payment application is loaded into preferably a secure element of the user mobile device a registration process is enabled. During this registration process on a server, an account of the user is created and a set of unpredictable numbers in quantity N is generated. This set of unpredictable numbers is transmitted to the mobile payment application for storage. The payment application uses the stored set of transaction unpredictable numbers for next N times transactions performed by the mobile payment application for transaction cryptogram calculation. The same transaction unpredictable number is recovered in the server side during the transaction authorization process.