Systems and methods for obtaining information on a key in the BB84 protocol of quantum key distribution are provided. The synchronous control circuit has several sets of power source ends and a set of induction ends. The arming comparator compares the integrated signal with a predetermined arming threshold and provides an armed signal indicative thereof. A sheet-metal machining of the first section of the sheet-metal strip is carried out using the first tool. In each cross-section perpendicular to the median plane, which cross-section passes through both opposing side cutting edges and also the at least one protrusion, no portion of the peripheral surface is inward of an imaginary line connecting the opposing side cutting edges. This head protecting airbag device simplifies the connection structure between the inflator and the airbag while retaining the quick expansion and inflation of the airbag.