Method for securing the time synchronization of an ethernet on-board network

A method for securing the time synchronization of an Ethernet on-board network of a motor vehicle, the method comprising:

• determining a delay time of a first signal on a first connecting path between a first control unit of the Ethernet on-board network and a second control unit of the Ethernet on-board network;
• determining a maximum speed of the first connecting path on the basis of the delay time;
• and
• determining a type of a transmission medium of the first connecting path on the basis of the maximum speed,
• wherein the determination of the delay time of a first signal and the determination of the maximum speed of the first connecting path and the determination of the type of a transmission medium of the first connecting path result in an entropy source being formed that is used to ascertain at least one dynamic key for the connecting path that is used to encrypt a time synchronization message for the connecting path.