Method for protecting a hands-free access and/or starting system of a vehicle by modifying the speed of signal reception

A method for protecting a hands-free access and/or starting system of a motor vehicle equipped with at least one electronic key, comprising:

• a step of emitting a coded identification request (38) emitted by an antenna of a central control unit of said system in the form of a coded interrogation signal (5, 5′) intended for the electronic key (K);
• a step of the electronic key (K) responding to give its identity (40);
• and
• a step of authorizing the access to (46) and/or starting of the vehicle (52),
• wherein said method furthermore comprises, before the step of authorizing the access to (46) and/or starting of the vehicle (52), additional protection steps consisting in:
• adapting the amplitude of at least one sinusoidal signal (Sa) received by the key in the coded identification request step (38) to reach an amplitude detection threshold (15, 16, 17) of the electronic key (K) as soon as possible;
• stopping said amplitude adaptation (42) as soon as the electronic key (K) is identified in the responding step (40);
• measuring (43) a sum (Tm, T′m) of durations of transmission of the coded interrogation signal (5, 5′) and of the response signal (7, 7′, 9) of the electronic key;
• comparing (44, 51) the sum of the measured durations (Tm, T′m) to a reference duration (S, S′);
• prohibiting the access to and/or starting of the vehicle when a difference between this sum of measured durations (Tm, T′m) and the reference duration (T2, T′2) is above a preset threshold (S, S′);
• and
• triggering alarms (50, 54) when the difference between this sum of measured durations (Tm, T′m) and the reference duration (T2, T′2) is above the preset threshold (S,S′).