'Verify that it was created by a trusted server (old-school),
or that it has an invitation backing the devicezone (new-school)'

@validate_via_booleans
def validate(self):

    if self.signed_by.is_trusted():
        return True
    invitation = get_object_or_None(ZoneInvitation, used_by=self.device, zone=self.zone)
    return ((self.device == self.signed_by) and invitation and invitation.verify())
