Hi,
you can see in AN5401:

So, when MASTER_ECU_KEY is loaded first time, blank key is used as an authorizing key:
uint32_t BLANK_KEY_VALUE[4] = {0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF}; //When key value is not written it is all FFs

Once the MASTER_ECU_KEY is loaded, it's used for authorization of other keys:

Also this could be useful:

Regards,
Lukas