A tachograph and a toll onboard unit as communication partners, which each have a data interface for a data communication via a vehicle data bus to which the communication partners are coupled. The tachograph and/or the toll onboard unit are implemented as a transmitter of data to ascertain a cryptographic check value as a function of user data, which are to be transmitted to the communication partner, and to transmit the cryptographic check value in addition to the user data to the communication partner. The toll onboard unit or the tachograph, respectively, as a receiver of data, is implemented to receive user data and the cryptographic check value associated with the user data from the communication partner and to check the received user data for corruption as a function of the received cryptographic check value.