A method for operating a tachograph having an application controller and a safety controller. A memory is associated with the application controller and a further memory is associated with the safety controller. An encrypted program code having at least one associated signature is made available to the safety controller. The encrypted program code is decrypted by means of the security controller and verified by the at least one signature. It is detected whether the decrypted program code (EPC) is intended for the application controller and/or for the security controller. Depending on what is detected, the program in the memory and/or the further program in the further memory is at least partially replaced by the decrypted program code and, depending on a result of the verification, the at least partially replaced program and/or the at least partially replaced further program is released for executing.