Represent MD5 hash as an integer

Question

In my user database table, I take the MD5 hash of the email address of a user as the id.

Example: email(example@example.org) = id(d41d8cd98f00b204e9800998ecf8427e)

Answer 1

Why ord()? md5 produce normal 16-byte value, presented to you in hex for better readability. So you can't convert 16-byte value to 4 or 8 byte integer without loss. You must change some part of your algoritms to use this as id.

Answer 2

There are good reasons, stated by others, for doing it a different way.

But if what you want to do is convert an md5 hash into a string of decimal digits (which is what I think you really mean by "represent by an integer", since an md5 is already an integer in string form), and transform it back into the same md5 string:

function md5_hex_to_dec($hex_str)
{
    $arr = str_split($hex_str, 4);
    foreach ($arr as $grp) {
        $dec[] = str_pad(hexdec($grp), 5, '0', STR_PAD_LEFT);
    }
    return implode('', $dec);
}

function md5_dec_to_hex($dec_str)
{
    $arr = str_split($dec_str, 5);
    foreach ($arr as $grp) {
        $hex[] = str_pad(dechex($grp), 4, '0', STR_PAD_LEFT);
    }
    return implode('', $hex);
}

Demo:

$md5 = md5('example@example.com');
echo $md5 . '<br />';  // 23463b99b62a72f26ed677cc556c44e8
$dec = md5_hex_to_dec($md5);
echo $dec . '<br />';  // 0903015257466342942628374306682186817640
$hex = md5_dec_to_hex($dec);
echo $hex;             // 23463b99b62a72f26ed677cc556c44e8

Of course, you'd have to be careful using either string, like making sure to use them only as string type to avoid losing leading zeros, ensuring the strings are the correct lengths, etc.

Answer 3

For a 32-bit condensation, a simple solution could be made by selecting 4 hex pairs (8 characters) of the MD5 hash, where each pair represents one byte, and then converting that with intval().

For an unsigned 32-bit Int:

$inthash = intval(substr(md5($str), 0, 8), 16);

For the positive value only of a signed 32-bit Int:

$inthash = intval(substr(md5($str), 0, 8), 16) >> 1;

This will likely only work for values up to 64-bit (8 bytes or 16 characters) for most modern systems as noted in the docs.

On a system that can accommodate 64-bit Ints, a splitting strategy that consumes the entire 128-bit MD5 hash as 2 Ints might look like:

$hash = md5($str);
$inthash1 = intval(substr($hash, 0, 16), 16);
$inthash2 = intval(substr($hash, 16, 16), 16);

Answer 4

Use the email address as the file name of a blank, temporary file in a shared folder, like /var/myprocess/example@example.org

Then, call ftok on the file name. ftok will return a unique, integer ID.

It won't be guaranteed to be unique though, but it will probably suffice for your API.

Answer 5

You could use hexdec to parse the hexadecimal string and store the number in the database.

Answer 6

Couldn't you just add another field that was an auto-increment int field?