Authentication method and device in a telecommunication network using a portable device

A method of authenticating the user (U) of a terminal (2) connected to an Internet type network, wherein the method being comprises the steps of:

• setting up (E15) a communications channel in a secure mobile telephony network (GSM) between mobile equipment (3) of said user (U) situated close to the terminal (2) and an authentication unit (1) connected to said Internet type network;
• the terminal (2) downloading (E30) via said Internet type network a digital code from said authentication unit (1);
• said mobile equipment (3) sending (E40) a sound signal to said authentication unit (1), the sound signal being generated (E35) by the terminal (2) on the basis of said digital code;
• and
• authenticating said user (U) on the basis:
• of the sound signal received (E40) via said mobile communications channel (GSM);
• and
• of an identifier (GSM_No) of said mobile equipment (3).