security.go
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
}
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
}
// Makes a node ID secure, in-place. The ID is 20 raw bytes.
// http://www.libtorrent.org/dht_sec.html