Cryptographic material is generated for a protocol for the encrypted transmission of media data between a subscriber device and a provider device. A first symmetric key of the subscriber device and the provider device is inserted in a symmetric key protection mechanism of a network protocol of a control layer to establish a communication session between the subscriber device and the provider device. A first time-variable parameter is transmitted from the provider device to the subscriber device. A second symmetric key for protecting the key management protocol is calculated by both the provider device and the subscriber device using a defined function depending at least on the first symmetric key and the first time-variable parameter.