The present invention relates to a method for authenticating a transmitter to a receiver, as well as for the protected transmission of messages; both the transmitter, as well as the receiver at least having a first common key; a random number, as well as at least one first partial code of a first code calculated from the random number with the aid of the first key from the receiver to the transmitter being transmitted in a synchronization message; the first partial code being checked by the transmitter; a first counter being generated by the transmitter; useful data, as well as a first partial counter of first counter and at least one second partial code of a second code calculated with the aid of a second key being transmitted by the transmitter to the receiver in a message; and the receiver checking the second partial code to verify the transmitter, as well as the transmitted message.