A method is disclosed for establishing a device-to-device communication session between mobile devices which are mutually connectable via a D2D communication channel (DDC) and individually connectable to a mobile network (MN), including preloading an initiation key on each of the mobile devices, the initiation key being associated with a validity period; and on each of the mobile devices: verifying a validity of the initiation key based on a current time; if the initiation key is deemed valid, generating a session key using the initiation key by using the initiation key in performing a key agreement procedure between the mobile devices over the D2D communication channel (DDC), the key agreement procedure resulting in the session key if the initiation key used by each of the mobile devices matches; and establishing the D2D communication session over the D2D communication channel (DDC) based on the session key.