Method for authorizing unlocking and/or starting of a motor vehicle and associated device

A method for identifying and authorizing unlocking or starting of a motor vehicle for a hands-free access system includes a central unit exchanging identification messages with identification units according to a communication procedure. The communication procedure includes emitting an identification message from the central unit to the identification units, emitting a message indicating a presence of the identification units having received the identification message to the central unit, emitting a selection message by the central unit to select one particular identification unit to reply to the identification message, and emitting an additional identification message, where at least one of the emitting of a message indicating the presence of the identification units and the emitting of the selection message is performed between emitting the identification message and the emitting of the additional identification message so as to enhance the security of interchange between the central unit and the identification units.