preOutput := (uint64(right) << 32) | uint64(left)
	byteorder.BePutUint64(dst, permuteFinalBlock(preOutput))
}

// DES Feistel function. feistelBox must be initialized via
// feistelBoxOnce.Do(initFeistelBox) first.
func feistel(l, r uint32, k0, k1 uint64) (lout, rout uint32) {
	var t uint32

	t = r ^ uint32(k0>>32)
	l ^= feistelBox[7][t&0x3f] ^
		feistelBox[5][(t>>8)&0x3f] ^
		feistelBox[3][(t>>16)&0x3f] ^