Method for authorising locking and/or starting a motor vehicle and associated device

The disclosure relates to a method for identifying and authorizing unlocking or starting of a motor vehicle for a hands-free access system, comprising a central unit (C) exchanging identification messages with identification units (B1, B2, B3) according to a communication procedure, said procedure including: a step that consists in emitting an identification message (CH) from the central unit (C) to the identification units (B1, B2, B3); a step that consists in emitting a message indicating the presence (ANTI) of said identification units (B1, B2, B3) having received the identification message (CH) to the central unit (C); a step that consists in emitting a selection message (SEL) by the central unit (C) to select one particular identification unit (B3) to reply to the identification message; wherein the procedure includes a step that consists in emitting an additional identification message (CH2), and wherein said step of sending a presence indication message (ANTI) and/or said step of sending a selection message (SEL) is performed between said step of sending an identification message (CH) and said step of sending the additional identification message (CH2) so as to enhance the security of interchange between said central unit (C) and said identification units. The invention also concerns a device for identifying and authorization unlocking and/or starting of a motor vehicle.