// for sBoxes[s][i][j] << 4*(7-s)
var feistelBox [8][64]uint32

var feistelBoxOnce sync.Once

// general purpose function to perform DES block permutations.
func permuteBlock(src uint64, permutation []uint8) (block uint64) {
	for position, n := range permutation {
		bit := (src >> n) & 1
		block |= bit << uint((len(permutation)-1)-position)
	}
	return
}

func initFeistelBox() {