code: func SecureNodeId(id *[20]byte, ip net.IP) {
  	crc := crcIP(ip, id[19])
  	id[0] = byte(crc >> 24 & 0xff)
  	id[1] = byte(crc >> 16 & 0xff)
  	id[2] = byte(crc>>8&0xf8) | id[2]&7
  }
docs: // Makes a node ID secure, in-place. The ID is 20 raw bytes.
  // http://www.libtorrent.org/dht_sec.html