PHP Generate an unique hash from an IPADDRESS - php

Question

I have a board and i want to disable registrations instead i want the users to be able to post freely with an unique ID based on their IPAddress
for example:
if "Joe" IP is 200.100.15.117, "Joe" nickname will become the hash function of 200.100.15.117
if "Joe" IP changes he will get another ID, that doesn't matter, i only want one unique ID per IPAddress
There are also two important things :
unique ID must be 8 characters long
hash should be secure, i don't want hackers be abble to crack my
users IPaddresses
I thought in using an MD5 function and then trim it to 8 chars, but how unique is that ? is there a better way ?

Answer 1

You can turn the ip address string to LONG(4 bytes) and do whatever you want next.
see the php function ip2long.

Answer 2

The thing with this is that you cant really diferentiate 2 persons on the same network, so if I post on there, and my dorm neighbour goes to the site, the site thinks he is me.
You should consider using cookies, these can easily be made unique, and AFAIK, can't be hacked remotely.
For the unique ID part, if you are storing the "users" in a database, you could just assign the primary key from your "user" table and use that in the cookie.
If it HAS to be 8 chars long, you could prepend 0's to the ID - e.g. 00000001, 00000002.... Atleast this way its unique, and 8 chars long-

Answer 3

EDIT According to OP comment on this answer.
Code below uses BC Math library to translate a MD5 hash into a Base-90 string. This converts a 32 char string into a 20 char one. This is far from desired 8 chars but it is the minimum string length using ASCII range (it is possible to increase the base to Base-94 by using chars ' " \ [space] but this does not affect to the string length and instead may cause problems while handling data).
$ip = '200.100.15.117';
$hash = md5($ip);
$chars16 = array(
'0' => 0, '1' => 1, '2' => 2, '3' => 3, '4' => 4, '5' => 5,
'6' => 6, '7' => 7, '8' => 8, '9' => 9, 'a' => 10, 'b' => 11,
'c' => 12, 'd' => 13, 'e' => 14, 'f' => 15
);
$base10 = '0';
for ($i = strlen($hash) - 1; $i > 0; $i--) {
$base10 = bcadd($base10, bcmul($chars16[$hash[$i]], bcpow(16, $i)));
}
$chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ,;.:-_+*?!$%&##~^=/<>[](){}`';
$base = (string)strlen($chars);
$baseX = '';
while (bccomp($base10, $base) === 1 || bccomp($base10, $base) === 0) {
$baseX = substr($chars, bcmod($base10, $base), 1) . $baseX;
$base10 = preg_replace('/\.\d*$/', '', bcdiv($base10, $base));
}
$baseX = substr($chars, $base10, 1) . $baseX;
echo $baseX; // Shows: 1BS[JdZf/7J$J{ud&r5i

Answer 4

You can salt the ip address, e.g.
$salt = "e45re%$#";
$ip = "200.111.123.111";
$hash = md5($ip.$salt);
$hash = substr($hash,0,8);